The effects of known anxiolytic agents and putative anxiolytic agents were assessed in mice in a fully automated 2-compartment light/dark test. Significant increases in lit area activities (e.g., time spent in the lit area, locomotor activity, rearing behavior) were used as possible indicators of anxiolytic-like action. The measurement found most consistent and useful for assessing antianxiety-like activity was the time mice spent in the lit area. The benzodiazepine, diazepam; the 5-HT1A agent, ipsapirone; and the 5-HT3 receptor antagonist, ondansetron, produced significant anxiolytic-like activity between doses of 1.0 to 10.0 mg/kg, 17.8 to 31.6 mg/kg, and 0.0001 to 1.0 mg/kg respectively. The 5-HT1A receptor agonist, 8-OH DPAT, also exhibited anxiolytic-like action between doses of 0.0005 to 3.16 mg/kg. In contrast, the peripheral 5-HT3 receptor agonist, N-phenylbiguanide; the antidepressant, imipramine; the neuroleptic, chlorpromazine; and the CNS stimulant, S(+)-amphetamine, did not display antianxiety-like activity. The positive results obtained for the three types of compounds (benzodiazepine, 5-HT1A, and 5-HT3) indicate that this fully automated light/dark apparatus may be useful for identifying known and putative anxiolytic agents.