Although oxytocin and its receptor have been identified in human ovary, its regulatory role in granulosa cell or corpus luteum function has not been clearly defined. To better understand oxytocin action in the human ovary, we have characterized the expression and function of oxytocin receptors in an immortalized human granulosa-lutein cell line, HGL5. Expression of oxytocin receptor mRNA was demonstrated by reverse transcriptase-polymerase chain reaction analysis, and by specific binding of an iodinated oxytocin antagonist (apparent dissociation constant of 131 ± 0.15 pM, and a B_max of 12 ± 0.5 fmol/μg DNA). Receptor levels were down-regulated by serum starvation, and rapidly up-regulated by serum restoration. Stimulation of protein kinase C activity increased oxytocin receptor levels in a concentration-dependent manner. Conversely, protein kinase C inhibition blocked up-regulation of oxytocin receptors. Treatment of cells with 10 nM oxytocin resulted in a rapid, transient increase in intracellular Ca² , and the response was blocked by an oxytocin antagonist. Because HGL5 cells secrete progesterone and estradiol in response to agents that elevate intracellular cAMP concentrations, we studied the effect of oxytocin on steroid production. Oxytocin enhanced the effects of forskolin on progesterone production. These results suggest that oxytocin augments the activity of luteotropins in vivo. Our studies are the first to show an ovarian cell line that expresses functional oxytocin receptors. These cells can serve as a useful model for studying oxytocin signal pathways and their cross-talk with respect to progesterone synthesis. These cells also will be useful in the analysis of mechanisms of oxytocin receptor regulation, including regulation of its gene.