In human placental explants cultured in vitro, dopamine inhibited human chorionic somatomammotropin (hCS) secretion into the culture media. In the control flasks, the level of hCS secretion was 130.5 +/- 7.8 micrograms/g tissue (n = 6). When 1 mM dopamine was added, hCS levels decreased to 80.2 +/- 11.5 micrograms/g tissue (P less than 0.01). Dopamine (5 and 10 mM) further lowered hCS levels. In contrast, 1 mM pimozide enhanced hCS secretion by 2-fold as compared to control levels (248.2 +/- 44.8 vs. 130.5 +/- 7.8, P less than 0.02). The simultaneous addition of dopamine did not alter the stimulatory effect of pimozide on hCS secretion. In separate experiments, arginine (1 and 5 mM) and somatostatin (1 microgram/ml culture media) did not alter hCS secretion from placental explants. These results suggest that hCS secretion is modulated by dopaminergic receptors.