This study sought to determine the effect of chlorpromazine (CPZ) on a physiological measure, the H-reflex recovery curve (HRRC) and to determine whether the effect is different in normal subjects and schizophrenic patients. Eleven normal control subjects and 14 patients were administered 12.5 and 25.0 mg CPZ by IM injection. On the average. HRRCs measured 90 min after the injections were lower compared to pre-injection levels in both groups, at both doses. In general, the higher dose was more effective in both groups. Half of the patients, however, failed to respond to 12.5 mg, indicating that some schizophrenics are less sensitive than normals to CPZ. These results indicate that excessive dopamine activity or sensitivity may underlie abnormally high HRRCs in unmedicated psychotic patients.