Uteri from non-pregnant and pregnant (Days 1-10) mice were examined for the presence of NADPH diaphorase (NADPH-d) activity by histochemical techniques. Macrophages positive for NADPH-d were observed in all uterine sections but appeared to migrate out of the implantation site and cluster in the mesometrium and interimplantation zones beginning on Day 4. NADPH-d activity was seen in the luminal and glandular epithelium and in several isolated fibers coursing through the myometrium. Many branches of the uterine artery also expressed activity, with the most intense staining in the vessels of the mesometrium. However, the most remarkable staining began on Day 6 within the primary decidual zone. When the stromal cells underwent decidualization, they began to show NADPH-d activity, with the pattern of activity matching the expanding area of decidualization. By Day 9 most of the decidual cell reaction had occurred and the mesometrial decidual staining began to decrease. However, the blood vessels and the cells surrounding the developing blood spaces continued to express activity, and heavy staining was evident within the antimesometrial decidua. No NADPH-d activity was seen in any of the trophoblast cells at any time, or in embryonic tissue, except on Day 8. NADPH-d has been used to identify nitric oxide (NO) synthase. Therefore, it may represent an NO-mediated paracrine control over decidual blood flow, myometrial quiescence, or immune response during pregnancy.