The interaction of diazepam and diphenylhydantoin on locomotor activity and rearing behavior was studied in mice. Pretreatment of mice with diphenylhydantoin (4.0 and 8.0 mg/kg) significantly reversed the stimulatory effects of low doses of diazepam and considerably increased the depressant effects of the benzodiazepine on locomotor activity and rearing. Neither diazepam (up to 4.0 mg/kg), diphenylhydantoin (8.0 mg/kg) alone, nor combined treatment with both drugs affected brain GABA levels and glutamic acid decarboxylase (GAD) activity at any dosage used. The present behavioral and biochemical data suggests that some of the pharmacological effects of diazepam need not be related to GABAergic mechanisms.