The resting membrane potential (RMP) of a neuron is set by a complex balance between charged ions, ion channels and transporters. Many of the ion channels have been identified at the molecular level. Missing from the molecular identification has been the voltage-insensitive background sodium ‘‘leak’’ conductance that depolarizes the RMP from the equilibrium potential of potassium and provides a crucial contribution to neuronal excitability One candidate for the molecular identity of this conductance is the protein NALCN. NALCN is a previously uncharacterized orphan member in the sodium/calcium channel family. It is widely expressed in the nervous system. My thesis project was designed to uncover the properties of NALCN and to find its functional roles, especially its contribution to the neuronal excitability as an ion channel. I found that NALCN formed a voltage-insensitive background sodium leak conductance. Such a conductance was detected in hippocampal neurons cultured from the wild-type mice but not the NALCN-/- mutant mice with targted disruption in the NALCN gene. The conductance in the NALCN-/- neurons was restored when NALCN cDNA was transfected. These results suggest that NALCN provides the major contribution to the voltage-insensitive background sodium leak conductance. We also discovered that the NALCN channel could be activated by the neuropeptides substance P (SP) and neurotensin (NT), and by lowering extracellular Ca2+ concentration ([Ca2+]e), both of which elicit excitatory effects on several types of neurons. In addition, we found that NALCN was activated by the two types of stimuli via distinct intracellular signaling pathways and the two had synergistic effects on each other. Finally, application of the neuropeptides or lowering [Ca2+]e did not excite hippocampal neurons cultured from the mutant, suggesting that the NALCN channel complex is a primary target for neuronal excitation control by the neuropeptides and extracellular Ca2+.