Recent evidence suggests that endogenous nitric oxide (NO) contributes to the modulation of hormonal secretion from the anterior pituitary gland according to the physiological state of the animal. In this study, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and specific neurochemical assay were used to asses possible changes of NO synthase (NOS) activity in the anterior pituitary during pregnancy and parturition in rats. The anterior pituitary showed (weak) NADPH-d activity throughout pregnancy. Parturition increased the number and intensity of NADPH-d-positive cells. The NADPH-d-positive cells co-localized with immunofluorescent LH-positive cells. No variation in NADPH-d activity was apparent during the various stages of the oestrous cycle. Furthermore, NOS activity during parturition increased significantly when compared with non-pregnant and pregnant rats. Increases in both specific activity and NADPH-d activity gradually decreased within 24 h post-partum, suggesting that NO may modulate anterior pituitary function during parturition.