Immunocytochemistry was used to localize endothelial (eNOS) and inducible (iNOS) nitric oxide synthase in human uterine tissues collected at various stages of the menstrual cycle, after exposure to exogenous progestagens, and in early pregnancy. Endothelial NOS-like immunoreactivity was detected in all specimens in endothelial cells lining blood vessels in the myometrium and endometrium, and in endometrial glandular epithelial cells. Inducible NOS-like immunoreactivity was also demonstrated in glandular epithelial cells. For both eNOS and iNOS there was considerable variation in the intensity of epithelial cell staining between samples, which was not related to the stage of the menstrual cycle at which the tissue was collected. Messenger RNA for eNOS and iNOS was detected by reverse transcription-polymerase chain reaction (RT-PCR) using total RNA purified from isolated endometrial gland fragments. Immunoreactivity for eNOS and iNOS was not present in endometrial stroma throughout the menstrual cycle, but iNOS-like immunoreactivity was seen in decidualized stromal cells both following treatment with exogenous progestagen (intrauterine L-norgestrel) and in tissues obtained in the first trimester of pregnancy. The detection of protein and mRNA for eNOS and iNOS in normal human endometrium suggests that NO may play a role in the local control of endometrial function.