Summary. The addition of acetylcholine or histamine (10− 7 to 10−4m), γ-aminobutyric acid, a dopamine agonist, and melatonin (10−7 to 10−5m) did not alter basal or LH-stimulated progesterone production (P>0.05). The addition of the specific β2-adrenergic agonist terbutaline and salbutamol did not significantly elevate progesterone production. Treatment of luteal cells with serotonin (5-HT), 10−6 to 10−4m, increased the production of progesterone (P<0.05). This stimulated production was inhibited by the addition of mianserin (10−5m, a 5-HT antagonist; P<0.05). Isoproterenol (10−7 to (10−4m) also resulted in significant increases in progesterone production (P<0.05). The combined treatments of 5-HT + LH, isoproterenol+ LH, or isoproterenol + 5-HT did not result in a further increase in progesterone above that observed in response to LH or isoproterenol alone (P>0.05). The isoproterenol-induced progesterone production could not be blocked by butoxamine (10−5m, a β2-antagonist), or practolol (10−5m, a β1-antagonist), but was inhibited by propranolol (10−5m, a general β-antagonist; P<0.05). The response to isoproterenol was unaffected by mianserin (10−5m). These results demonstrate a possible role for 5-HT in the regulation of steroidogenesis by the corpus luteum of the cow. Furthermore, these results suggest that serotonin-induced progesterone production is a receptor-mediated event.