In view of the fact of the high concentration of neuronal nitric oxide synthase (NOS) in the supraoptic and paraventricular nuclei as well as the posterior pituitary, it has been supposed that nitric oxide (NO) may be involved in the regulation of hormone secretion from these sites. In the present study, L-arginine, the precursor of NO, and N(omega)-nitro-L-arginine methyl ester (L-NAME), the NOS inhibitor, were intracerebroventricularly (i.c.v.) administered into the pentobarbital anesthetized rats. The results showed that plasma arginine vasopressin (AVP) concentration increased after the injection of L-arginine (1.0 mg in microliters, i.c.v.) and decreased after the L-NAME (270 micrograms in 5 microliters, i.c.v.). The elevated plasma AVP level in response to short-term hypotension induced by intravenous infusion of sodium nitroprusside was also significantly reduced by i.c.v. administration of L-NAME. The results indicate that NO acts as a stimulating factor to both the basal and reflex release of AVP, opposing the view that NO plays an inhibitory role in the regulation of AVP secretion.