BACKGROUND Uncontrolled evidence suggests that divalproex administered via the oral loading strategy of 20 mg/kg/day may produce clinically significant antimanic response within 3 days of treatment in some patients. We conducted a prospective study to compare the antimanic response of divalproex oral loading with that of haloperidol in the initial treatment of acute psychotic mania. METHODS After a < or = 1-day screening period, 36 consecutive hospitalized patients with bipolar disorder, manic or mixed phase and with psychotic features, were randomly assigned to receive either divalproex 20 mg/kg/day or haloperidol 0.2 mg/kg/day for 6 full days, without other psychotropic agents except lorazepam up to 4 mg/day for management of agitation. Serum valproate concentrations were measured after 1 day of treatment. Response was measured daily by a blind rater using the Young Mania Rating Scale and the Scale for Assessment of Positive Symptoms. RESULTS Divalproex oral loading and haloperidol were equally effective in acutely reducing manic and psychotic symptoms. The greatest rate of improvement for both drug regimens occurred over the first 3 full days of treatment. Side effects were infrequent and minor for both treatments, except for extrapyramidal side effects which were significantly more common with haloperidol. CONCLUSIONS Divalproex oral loading may produce rapid onset of antimanic and antipsychotic response comparable to that of haloperidol and with minimal side effects in the initial treatment of acute psychotic mania in a subset of bipolar patients.