Plasma and red blood cell levels of haloperidol were measured in psychotic patients after an acute dose of haloperiodol and during treatment with fixed doses for several weeks. There was wide variability of haloperidol plasma and red blood cell levels at a given dose and a nonlinear relationship between haloperidol dose and blood levels. The red blood cell/plasma ratio (0.40) had a narrow range among different patients. Haloperidol blood levels were not correlated with the age of the patient. Concomitant administration of benztropine mesylate did not decrease plasma haloperidol levels. Steady state plasma or red cell haloperidol levels could not be predicted from levels 24 hours after the acute dose.