It has been reported that arginine vasopressin (AVP) plays a thermoregulatory action, but very little is known about the mechanisms involved. In the present study, we tested the hypothesis that nitric oxide (NO) plays a role in systemic AVP-induced hypothermia. Rectal temperature was measured before and after AVP, AVP blocker, orN ^G-nitro-l-arginine methyl ester (l-NAME; NO synthase inhibitor) injection. Control animals received saline injections of the same volume. The basal body temperature (T_b) measured in control animals was 36.53 ± 0.08°C. We observed a significant (P < 0.05) reduction in T_b to 35.44 ± 0.19°C after intravenous injection of AVP (2 μg/kg) and to 35.74 ± 0.10°C after intravenous injection ofl-NAME (30 mg/kg). The systemic injection of the AVP blocker [β-mercapto-β,β-cyclopentamethylenepropionyl¹,O-Et-Tyr²,Val⁴,Arg⁸]vasopressin (10 μg/kg) caused a significant increase in T_b to 37.33 ± 0.23°C, indicating that AVP plays a tonic role by reducing T_b. When the treatments with AVP and l-NAME were combined, systemically injected l-NAME blunted AVP-induced hypothermia. To assess the role of central thermoregulatory mechanisms, a smaller dose ofl-NAME (1 mg/kg) was injected into the third cerebral ventricle. Intracerebroventricular injection ofl-NAME caused an increase in T_b, but when intracerebroventricular l-NAME was combined with systemic AVP injection (2 μg/kg), no change in T_b was observed. The data indicate that central NO plays a major role mediating systemic AVP-induced hypothermia.