OBJECTIVE To explore the role of nitric oxide (NO) in arginine vasopressin (AVP)-induced hypothermia. METHODS Colonic temperature was measured at 30 min intervals with a digital thermometer. Effects of central and peripheral administration of nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) on AVP-induced hypothermia were observed in rats. RESULTS (1) Intravenous injection (i.v.) of AVP (4 microg x kg(-1)) and L-NAME (30 mg x kg(-1)) produced a significant drop in body temperature, respectively. Coadministration of L-NAME i.v. with AVP i.v. did not significantly influence the magnitude of AVP-induced hypothermia. (2) Intracerebroventricular (i.c.v.) injection of L-NAME (1 mg x kg(-1)) caused a significant increase in body temperature, but when the treatments with AVP and L-NAME were combined, i.c.v. injection of L-NAME markedly blocked the hypothermic effect of AVP intravenous injection. CONCLUSIONS Endogenous NO in the central nervous system plays a major role in AVP-induced hypothermia. In addition, central NO seems to play a tonic thermoregulatory role by reducing normal body temperature because an increase in body temperature was observed after treatment with NOS inhibitor L-NAME.