OBJECTIVE To study localization, distribution and activity of nitric oxide synthase (NOS) in the human placenta and to speculate the action of nitric oxide (NO) during pregnancy. METHODS NADPH-diaphorasc histochemical method was used to indicate the distribution, localization and activity of NOS in the placentae of normal term pregnancy (n = 21). Severe pregnancy induced hypertension (PIH) (n = 15), intrauterine growth retardation (IUGR) (n = 2) and villi of early pregnancy (n = 16). RESULTS The NOS reactive product called blue formazan was found in most of syncytiotrophoblast (STr) and located in top of cells in severe PIH and IUGR placentae. In normal term placentae the blue formazan was found in STr and located in base of cells. The number of blue formazan was more than that of PIH and IUGR placentae. Endothelial cells of most capillaries of the terminal villi in severe PIH and IUGR placentae were found to be deposited with blue formazan, but in normal term pregnancy, blue formazan was found only on rare occasion. CONCLUSIONS Distribution and localization of NOS in placentae of normal term pregnancy, severe PIH and IUGR cases and early villi are specific. The NOS activity of STr in severe PIH placentae and IUGR placentae are lower than that of normal term placentae. The distribution and localization of NOS within the STr in severe PIH placentae and IUGR placentae are different from those in normal term placentae. The low activity of NOS secreted by placenta may be relative to the pathogenesis of PIH and IUGR.