Anapyrexia (a regulated decrease in body temperature) is a response to hypoxia that occurs in organisms ranging from protozoans to mammals, but very little is known about the mechanisms involved. Recently, it has been shown that the NO pathway plays a major role in hypoxia-induced anapyrexia. However, very little is known about which of the three different nitric oxide synthase isoforms (neuronal, endothelial, or inducible) is involved. The present study was designed to test the hypothesis that neuronal nitric oxide synthase (nNOS) plays a role in hypoxia-induced anapyrexia. Body core temperature (T_c) of awake, unrestrained rats was measured continuously using biotelemetry. Rats were submitted to hypoxia, 7-nitroindazole (7-NI; a selective nNOS inhibitor) injection, or both treatments together. Control animals received vehicle injections of the same volume. We observed a significant (P < 0.05) reduction in T_c of ∼2.8°C after hypoxia (7% inspired O₂), whereas intraperitoneal injection of 7-NI at 25 mg/kg caused no significant change in T_c. 7-NI at 30 mg/kg elicited a reduction in T_c and was abandoned in further experiments. When the two treatments were combined (25 mg/kg of 7-NI and 7% inspired O₂), we observed a significant attenuation of hypoxia-induced anapyrexia. The data indicate that nNOS plays a role in hypoxia-induced anapyrexia.