The neurohypophyseal hormone oxytocin (OT) regulates biologic functions in both peripheral tissues and the central nervous system. In the central nervous system， OT influences social processes， including peer relationships， maternal-infant bonding， and affiliative social relationships. In mammals， the nonapeptide OT structure is highly conserved with leucine in the eighth position (Leu-OT). In marmosets ()， a nonsynonymous nucleotide substitution in the gene codes for proline in the eighth residue position (Pro-OT). OT binds to its cognate G protein-coupled receptor (OTR) and exerts diverse effects， including stimulation (G) or inhibition (G) of adenylyl cyclase， stimulation of potassium channel currents (G)， and activation of phospholipase C (G). Chinese hamster ovary cells expressing marmoset or human oxytocin receptors (mOTRs or hOTRs， respectively) were used to characterize OT signaling. At the mOTR， Pro-OT was more efficacious than Leu-OT in measures of G activation， with both peptides displaying subnanomolar potencies. At the hOTR， neither the potency nor efficacy of Pro-OT and Leu-OT differed with respect to G signaling. In both mOTR- and hOTR-expressing cells， Leu-OT was more potent and modestly more efficacious than Pro-OT in inducing hyperpolarization. In mOTR cells， Leu-OT-induced hyperpolarization was modestly inhibited by pretreatment with pertussis toxin (PTX)， consistent with a minor role for G activation; however， the Pro-OT response in mOTR and hOTR cells was PTX insensitive. These findings are consistent with membrane hyperpolarization being largely mediated by a G signaling mechanism leading to Ca-dependent activation of K channels. Evaluation of the influence of apamin， charybdotoxin， paxilline， and TRAM-34 demonstrated involvement of both intermediate and large conductance Ca-activated K channels.