The precise mechanism of set-point regulation in hypothalamus was not elucidated. Nitric oxide synthases (NOS) were detected in hypothalamus, however, the roles of NO in hypothalamus was not fully studied. So, we tested the effects of NO on body temperature because preoptic-anterior hypothalamus was known as the presumptive primary fever-producing site. NO donor sodium nitroprusside (SNP, 4 nmol, i.c.v.) elicited marked febrile response, and this febrile response was completely blocked by indomethacin (a cyclooxygenase inhibitor). But, ODQ (selective guanylate cyclase inhibitor, 50μg, i.c.v.) did not inhibit fever induced by SNP. The cyclic GMP analogue dibutyryl-cGMP (100μg, i.c.v.) induced significant pyreses, which is blocked by indomethacin. NG-nitro-L-arginine methyl ester (L-NAME, non selective NOS inhibitor) inhibited fever induced by interleukin-1β (IL-1β, 10 ng, i.c.v.), one of endogenous pyrogens. These results indicate that NO may have an important role, not related to stimulation of soluble guanylate cyclase, in the signal pathway of thermoregulation in hypothalamus.