Diazepam lowered homovanillic acid levels in brain regions from untreated and haloperidol-treated rats. However, there was evidence of a biphasic effect in that 2 mg/kg was more effective than 10 mg/kg under several conditions. Diazepam was more effective in restoring regional HVA toward control levels following doses of haloperidol in the low clinical range (0.1 to 0.2 mg/kg) than following higher doses. With respect to maximizing dopamine receptor blockade and minimizing compensatory presynaptic activity in the acute treatment of psychotic conditions, our data provide some support for the use of modest doses of neuroleptics and benzodiazepines.