The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) or intracerebroventricular (i.c.v.) routes or was infused directly into the dorsal raphe nuclei (DRN). Subcutaneous (1.25-5.0 mg/kg) or i.c.v. (0.25-1.0 mg) administration of L-NAME increased waking (W) and reduced slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) during the first 3 h of recording. On the other hand, direct application of L-NAME into the DRN (50.0-150.0 microg) induced an increment of W and a reduction of SWS without suppressing REMS. Values of W and SWS were significantly different compared with those of controls during the 6-h recording period. The effects of L-NAME observed after s.c. or i.c.v. administration confirm previous studies in rabbits and rats, in which the NOS inhibitor reduced sleep and increased W in a dose-dependent manner. It is possible that REMS suppression after L-NAME could be related to a reduction of acetylcholine release in areas critical for REMS promotion. A decrease in gamma-aminobutyric acid (GABA) release after nitric oxide synthesis inhibition could play a role in the reduction of SWS.