The aim of this study was to investigate the influence of the nitric oxide/nitric oxide synthase (NO/NOS) system on the release of endothelin-1 (ET-1) in human endometrial cells. Human endometrial stromal cells in secretory phase were incubated for 72 h in serum-free RPMI 1640 medium in the absence or presence of different concentrations of interleukin-1beta (IL-1beta) and NG-monomethyl-L-arginine (LNMMA), a specific competitive inhibitor of NOS. ET-1 released from the cultured cells into the medium was determined by specific RIA. In all the experiments at various times, IL-1beta significantly increased the release of ET-1. LNMMA significantly attenuated the release of ET-1 when the cells were cultured with both IL-1beta and LNMMA, but LNMMA alone had no effect on ET-1 release. These results suggest that the NO/NOS system in human endometrium is involved in the regulation of ET-1 release via IL-1beta secretion. It can also be inferred that NO and ET-1 control the functions of endometrium in close association with IL-1beta.