The castration of young piglets on commercial swine farms is a practice dating back thousands of years, with the earliest mentions of piglet castration as a regular practice in swine farming and breeding dating back Mediterranean area 1100-800 BCE (Morlein, 2020). In modern times, the primary reason for castration of male piglets is the occurrence of boar taint, an offensive odor and taste that occurs when cooking and eating meat from intact male pigs (Bonneau & Weiler, 2019). The two main components associated with boar taint are androstenone and skatole, lipophilic compounds that accumulate in the adipose tissue of growing animals (Claus et al., 1994). In addition to elimination of boar taint, early castration also reduces aggression and sexual behaviors, such as mounting, in male pigs which have the potential to cause injury and affect welfare (Compassion in World Farming, 2022). These behaviors increase with the onset of puberty and production of sex hormones (Compassion in World Farming, 2022) and are all but eliminated by castrating boars at a young age. During piglet castration, which is typically performed between two and seven days of age, it is standard and accepted practice to not provide any anesthesia or analgesia, even though it is standard of care to for companion animals undergoing castration to be anesthetized for the procedure and for them to receive balanced, multimodal analgesia (Griffin et al., 2016). According to a focus group of swine veterinarians, there are three barriers to providing pain relief during castration of piglets: the lack of approved products validated for efficacy, economic limitations and challenges, and deficient guidelines and training for veterinarians to develop protocols (Wagner et al., 2020). Castration of piglets is associated with physiological and behavioral changes that indicate stress and pain, making it a significant welfare concern. First, there are numerous studies that have shown that surgical castration causes a significant and marked increase in cortisol concentrations, while showing that handling alone only causes slight, non-significant cortisol increases (Carroll et al., 2006; Webb, 2020). This suggests that the increase in cortisol in response to castration is predominantly due to the pain associated with castration and not the stress of handling. In another study by (Hay et al., 2003), castration was shown to induce acute activation of the hypothalamic–pituitary–adrenal axis (HPA) and of the sympathetic nervous system (SNS), which are both activated in times of severe distress. In addition to cortisol, DHEA-S concentration can be used to indicate stress. In the steroid pathway, decreases in DHEA-S in the presence of an increased cortisol level indicate that production is halted in order to achieve adequate serum cortisol concentrations necessary to cope with stressors (Carroll et al., 2006). A study that evaluated DHEA-S levels in conjunction with cortisol showed that castrated piglets had a significant decrease in DHEA-S and significant increase in cortisol, while in non-castrated piglets, although cortisol concentrations increased, the stress of simulated castration was evidently not sufficiently severe to elicit a change in the steroid pathway to increase cortisol production at the expense of DHEA-S (Carroll et al., 2006). Other physiologic changes indicative of stress in castrated piglets include increased lactate concentration (Prunier et al., 2005) and heart rate elevations (White et al., 1995). In addition to physiological changes, there are numerous studies describing the behavioral changes that occur in piglets following castration, including a reduction in sucking time, a decrease in activity, an increase in lying time, abnormal postures, tail position, tail wagging, and vocalizations (Hay et al., 2003; McGlone et al., 1993; Miller et al., 2023; Sutherland et al., 2010, 2012). In choosing an analgesic to evaluate for efficacy, the logical first choice is the nonsteroidal anti-inflammatory drug (NSAID) flunixin meglumine. It is the only FDA-approved NSAID for use in pigs and is commonly used for pyrexia associated with swine respiratory disease (Nixon et al., 2020). Its mechanism of action is inhibiting the enzyme cyclooxygenase (COX), which plays a role in the arachidonic acid cascade by converting it to prostaglandins. Therefore, flunixin blocks the formation of prostaglandins and other inflammatory mediators by interrupting the activity of COX (Hassan et al., 2016). In addition to being labeled for pyrexia in swine, it is also labeled for pyrexia and inflammation in cattle and inflammation and pain associated with musculoskeletal disorders and colic in horses (Banamine [Package Insert], 2021). The pharmacokinetics of flunixin meglumine following intramuscular (IM) administration in pre-wean piglets was investigated by (Kittrell et al., 2020). In this study, 39 piglets received the labeled dose of flunixin meglumine (2.2 mg/kg) IM and plasma was collected at 27 timepoints over nine days. They found that peak plasma concentrations following IM administration is 30 minutes, which is why this timepoint was chosen in the current study. They found that the half-life is 9.12 hours with a Cmax of 6.543 μg/ml. Using the therapeutic plasma concentration of 0.95 ug/ml, determined in other species (Kendall et al., 2023), flunixin meglumine will stay above therapeutic levels for just over 18 hours in piglets. Pain is an important indicator of animal welfare, and the ability to detect pain in animals in order to provide adequate analgesia is of paramount importance. Physiologic measures are relevant, such as heart rate, respiratory rate, pupillary diameter, and blood pressure, but it is not always practical to measure these parameters (Hernandez-Avalos et al., 2019). Additionally, these parameters are also effected by fear and stress, such as caused by handling, and variations are not as evident when evaluating chronic pain (Hernandez-Avalos et al., 2019). As such, behavioral changes have been recognized as indicators of pain in animals as pain generates changes in behavior that indicate the presence, location, and severity of pain (Reid et al., 2018). Behavior may be measured using continuous coding which has been shown to be better than scan sampling. There is also the possibility of using pain scales to replace time consuming coding of videos. There are multiple validated pain scales used for companion animal species. These include the Glasgow Composite Measuring Pain Scale (CMPS) in dogs (Holton et al., 2001), the University of Melbourne Pain Scale used in dogs, (Firth & Haldane, 1999), and several others used in dogs and/or cats (Duke-Novakovski et al., 2016)(Brondani et al., 2011). While having validated pain scales for companion animals in inarguably important, it is equally important to have validated ways of assessing pain in farm animal species to provide better welfare. For pigs specifically, there is the Piglet Grimace Scale, which has been shown to be potentially effective with the need for further validation (Vullo et al., 2020). While there are other scales and ways of determining pain in piglets, there is only one validated scale for use in piglets. The Unesp-Botucatu pig composite acute pain scale (UPAPS) has been validated in pigs and piglets using specific pain or discomfort and maintenance behaviors (Luna et al., 2020; Robles et al., 2023). All pain-related behaviors composing the UPAPS have already been associated with pig pain conditions in previous ethograms, making it a valuable tool that brings together information from multiple sources, creating a scoring system that covers a wide variety of pain related behaviors displayed by piglets. Specifically, the study by (Robles et al., 2023) validated the UPAPS in piglets undergoing castration. For these reasons, this is the scale used for pain scoring in this study. The UPAPS includes six behavioral categories: posture, interaction and interest in surroundings, activity, nursing, attention to the affected area, and miscellaneous behaviors. While the UPAPS has been validated to detect pain in piglets following castration, it has not yet been used to evaluate the influence of analgesics on the behaviors included in the scale. The objectives of the current study are to contribute to the body of knowledge regarding castration of piglets, administration of analgesics, and the behavioral outcomes. The UPAPS will be used to score piglets following routine castration that were placed into one of three groups: those did not receive analgesics, those that received a single injection of flunixin meglumine at the time of castration, and animals that underwent a sham procedure (female piglets). The hypothesis is that piglets that receive flunixin meglumine will have significantly lower behavioral scores compared to those that did not receive analgesia and will have scores like the sham animals.