Haloperidol (HL) and reduced haloperidol (RH) plasma concentrations were measured in geriatric patients (n = 45) and schizophrenic patients (n = 8). In the elderly patients, HL doses were 1–4 mg/day while only 2 mg/day was used in the schizophrenics. At HL 2 mg/day dose in both age groups, mean plasma HL levels were approximately twice as high in the elderly patients compared to the schizophrenics (1.39 ± 0.82 vs. 0.56 ± 0.23 ng/ml, p < 0.02). RH plasma concentrations were almost 5 times greater in the elderly patients (0.54 ± 0.35 vs. 0.09 ± 0.05 ng/ml, p < 0.0001). These results suggest that HL plasma concentrations in the elderly are greater than in adult schizophrenic patients treated with similiar HL