Single or repeated subcutaneous administrations of naloxone in doses of up to 4 mg∙kg−1 did not alter the time for onset or the duration of ethanol- or pentobarbital-induced narcosis in rats. An increase in the naloxone dosage to 50 mg∙kg−1 s.c. resulted in a small reduction of ethanol-induced sleep duration. Repeated i.p. or i.v. administrations of much higher doses of naloxone (300–400 mg∙kg−1 and 120–180 mg∙kg−1, respectively) did produce significant reductions in ethanol-induced narcosis. However, similar naloxone doses, when administered alone, i.v., had marked convulsant effect. It appears that naloxone, in low doses, is not an effective antidote for narcosis caused by ethanol or pentobarbital at the ethanol and pentobarbital doses tested, whereas the antagonism of ethanol-induced sleep by high doses of naloxone may be due to the analeptic action of this drug.