Extracellular concentration of nitrite (NO2-), an oxidized product of nitric oxide (NO), was measured consecutively in the dorsal reg ion of the rat suprachiasmatic nucleus (SCN) by means of in vivo microdialysis. The NO2- concentrations in the dialysates showed robust circadian rhythm under a 12:12 h light/dark cycle and were higher during the dark phase than during the light phase. When the rats were transferred to constant darkness, the 24 h rhythm of NO2- persisted without damping the amplitude. The NO; level was significantly lowered by an injection of NO synthase inhibitor (N-G monomethyl-L-arginine, 10 mg/ka i.p.). These findings indicate that the daily fluctuation of NO2- in the dorsal region of the SCN, which represents endogenous rhythm of NO, is regulated independently of photic inputs into the SCN and may be related to the circadian clock functions. (C) 2001 Published by Elsevier Science Ireland Ltd.