The effects of various doses of ethanol (ETOH) on spontaneous locomotor activity (SLMA) in mice were measured using photocell activity chambers. Of the 4 i.p. doses injected, the 2 lowest doses (0.5 and 1.0 g/kg) stimulated SLMA, the next higher dose (2.0 g/kg) produced a biphasic effect of depression followed by stimulation, and the highest dose (4.0 g/kg) depressed SLMA. The mechanism of the biphasic effect of the 2.0 g/kg dose was studied in tests with central catecholamine antagonists at various doses 30 min before ETOH. Doses of 5, 10, and 20 mg/kg of propranolol, a beta-receptor blocker, significantly antagonized the depressant effect of ETOH but had no influence on the stimulant effect. High doses (10 and 20 mg/kg) of phentolamine, an alpha-receptor blocker, significantly antagonized the stimulant phase of ETOH action but had no significant effect on the depressant phase. All doses (0.062-0.250 mg/kg) of spiroperidol, a dopaminergic blocking drug, significantly enhanced the SLMA depression produced by ETOH. These results indicate that the SLMA-depressant effect of ETOH may be mediated by central "beta-type" receptors, that the SLMA-stimulant effect of ETOH may be mediated by central "alpha-type" receptors, and that at least part of ETOH's action may be due to dopaminergic mechanisms.