The human placental syncytiotrophoblast is derived from differentiating cytotrophoblasts and is in contact with maternal blood. This endothelial function positions the trophoblast to regulate maternal-fetal exchange and to influence circulatory dynamics through paracrine interactions in the placenta. Two isoforms of nitric oxide synthase (NOS) are expressed in placenta, and northern analysis, reverse transcription-polymerase chain reaction (RT-PCR), and immunocytochemistry were used to correlate expression of the type II, inducible NOS (iNOS) and the type III, endothelial NOS (eNOS) with state of differentiation in cultured trophoblast from term placentae. It was also tested whether cytokines known to induce NOS in other cell systems would induce iNOS in human trophoblast. The mRNA for eNOS was detected by RT-PCR, but not by Northern analysis, in cultures grown for 24 h when cytotrophoblasts were dominant. In contrast, eNOS mRNA was abundant in cultures grown for 72 h when syncytiotrophoblast was present. Immunocytochemical staining for eNOS protein showed specific fluorescence in a few cells in cultures at 24 h, but the vast majority of cells expressed eNOS at 72 h. The iNOS isoform was expressed neither basally in any trophoblast culture nor was this isoform induced in cultures exposed to interleukin-1, tumour necrosis factor-alpha, interferon-gamma and lipopolysaccharide. The in vitro pattern of trophoblast eNOS expression models the in vivo pattern of eNOS expression described for villous trophoblast. The results suggest that eNOS plays a role in human trophoblast differentiation and function.