The effect of somatostatin (SRIF) on human chorionic somatomammotropin (hCS) secretion was studied in human placental explants cultured in vitro. In the experimental flasks, SRIF was added in a concentration of 10, 100, and 1000 ng/ml media; hCS levels measured by RIA were not different from those found in the control flasks. In separate experiments, we investigated the action of SRIF on hCG secretion by a human malignant choriocarcinoma cell line maintained in tissue culture. SRIF (1000 ng/ml) did not inhibit basal or dibutyryl cAMP-induced stimulation of hCG secretion. These results suggest that somatostatin does not suppress hCS or hCG release in vitro from normal or malignant trophoblast, respectively.