Diabetes mellitus (DM) is a common metabolic disorder with complications including nephropathy, cardiomyopathy, neuropathy and infertility. We investigated nitric oxide synthase (NOS) expression in uterine tissue of diabetic rats. The rats were assigned randomly to four groups of ten: group C, control; group 1, diabetic for 1 month; group 2, diabetic for 2 months; group 3, diabetic for 3 months. DM was induced by streptozotocin injection. nNOS, iNOS, eNOS expressions and TUNEL staining were assessed in uterine tissue and luteinizing hormone (LH), follicle stimulating hormone (FSH) and estrogen were measured in blood samples. The uterine epithelium, connective tissue of the endometrium, myometrium wall thickness and number of apoptotic cells were decreased in groups 1 - 3 compared to group C. nNOS and eNOS expression was increased in diabetic groups, but iNOS expression was similar to group 1. FSH was increased in group 2, but serum LH and estrogen levels were unchanged among groups. DM damages uterine tissue, and NOS, especially nNOS and eNOS, contributes to this damage.