Nitric oxide (NO), a highly reactive free radical is involved in vasodilation, neurotransmission, hormone secretion, and reproduction. Since all known nitric oxide synthase (NOS) isoforms possess NADPH-diaphorase (NADPH-d) activity, NADPH-d histochemistry was used as a commonly accepted procedure for NOS identification. The aim of our study was to determine the cellular localization of NADPH-d, eNOS, and iNOS in the porcine uterus and the correlation between NADPH-d and NOS activity in the early, middle, late luteal, and follicular phase of the estrous cycle. Light-microscopic observations of the sections revealed the differential expression of the NADPH-d in the analyzed stages of the estrous cycle. The most intense staining was observed in the luminal epithelium in the late luteal phase and in some groups of the endometrial glands in all studied stages. Positive reaction was also found in the endothelial cells of blood vessels and in the myometrium itself. Immunostaining for eNOS was observed in the luminal and glandular epithelium in all studied stages, but no clear fluctuations were observed. The endothelium of both endometrial and myometrial blood vessels displayed pronounced eNOS immunostaining. Strong iNOS staining was observed in the luminal epithelium in the late luteal and follicular phase and in selected groups of endometrial glands. Thus, only NADPH-d and iNOS undergo cyclic changes in the studied stages of the estrous cycle. The differential expression of NADPH-d/NOS in the porcine uterine horn during the estrous cycle suggests a role for NO in modulating uterine function.