The effect of luteinizing hormone-releasing factor (LRF) on secretion of human chorionic gonadotropin (hCG) by the human placenta in culture was studied. A specific stimulation of hCG secretion was observed. The stimulation of hCG production correlated with the LRF dose in the culture medium. On the other hand, the secretion of human chorionic somatomammotropin was not affected. These data demonstrate a specific action of LRF on placental production of hCG in vitro.