Nitric oxide (NO) has been shown to be an important modulator of the febrile response to pyrogens and to psychological stress. In the present study, we aimed to identify the nitric oxide synthase (NOS) isoform (neuronal or inducible, nNOS and iNOS, respectively) involved in restraint stress fever. Colonic temperature (Tc) was measured in unanesthetized rats before and after treatment with the more selective nNOS inhibitor 7-nitroindazole or with the selective iNOS inhibitor aminoguanidine (AG) under unrestrained or restrained conditions. Intraperitoneal injection of AG (25 or 50 mg/kg) did not affect restraint fever, indicating that iNOS is unlikely to be involved in restraint fever. On the other hand, intraperitoneal injection of 7-nitroindazole (25 mg/kg) significantly attenuated the rise in the Tc caused by restraint stress, whereas it caused no change in Tc of euthermic animals. These data show that NO produced by nNOS plays an important role in the genesis of restraint stress-induced fever.