Leptin, the 16-kDa protein product of the obese gene, was originally seen as an adipocyte-derived signaling molecule. Recently, it has been suggested to be involved in some functions during pregnancy, particularly in the placenta. In the present study, we investigated the role of leptin in the secretion of hCG, progesterone, and interleukin-6 (IL-6) by human term trophoblast cells in culture. Placentae were obtained from cesarean sections following uncomplicated pregnancies and used immediately after delivery. Leptin, hCG, progesterone, and IL-6 were measured by ELISA, RIA, and immunoradiometric assay in the cultured media of trophoblast cells cultured for 48 and 96 h. Leptin mRNA expression in these cultures was determined by reverse transcription-polymerase chain reaction. Recombinant human leptin added to primary cultures of human term placental trophoblast cells showed a stimulatory effect on hCG and IL-6 secretion and an inhibitory effect on progesterone secretion. Primary cultures of term trophoblast cells expressed leptin mRNA. All these findings suggest a role for leptin in human placental endocrine function.