Most of the common arrhythmias and conduction disturbances have been reported in domestic mammals. Prevalence data, however, are not available from unselected population samples, and little information is at hand on long-term studies of individual cases. A discussion of some of these disturbances, therefore, must necessarily rely more heavily upon clinical impression than is desirable. Data have been published on normal electrocardiograms in different species, which, coupled with published case series, individual case reports, and observations made at the Heart Station of the University of Pennsylvania School of Veterinary Medicine, provide the basis for this presentation. This information suggests that certain arrhythmias and conduction disturbances occur commonly in some species and should be regarded as normal variants. In animals, departures from the normal regular sinus rhythm characteristic of adult humans apparently result from a relatively greater vagal influence on the cardiac pacemaker and conduction tissues. This certainly accounts for respiratory sinus arrhythmia in dogs. In horses, high resting vagal activity is considered accountable for sinus arrhythmia (respiratory and non-respiratory), wandering pacemaker, varying P-R interval, SA block, and incomplete AV block with dropped beats. Although these arrhythmias and conduction disturbances may occur with primary heart disease, or disease primarily affecting vagal activity, their frequent occurrence in the absence of detectable heart disease and also their disappearance with increased heart rate following exercise, excitement, or atropine, support the view that vagal activity rather than primary heart disease accounts for the majority of occasions in which they are observed. Frequently, more than one of the preceding arrhythmias are observed in one individual in the same or subsequent tracings; therefore frequency data often reflect some overlapping. It is useful, however, to present them individually when discussing more specific information.