Concentrations of promazine in plasma, plasma water, red blood cells, and urine were measured after oral administration of the drug to six patients during and after apparent recovery from the acute phase of viral hepatitis B. None of the promazine pharmacokinetic parameters were significantly different during and after the acute phase; these parameters included clearance, free drug clearance, metabolic clearance, volume of distribution, distribution and elimination half-life values, plasma protein binding, and per cent excreted in the urine. During the acute period of the illness, SGOP, SGPT, alkaline phosphatase, and total bilirubin were increased in all patients; they returned to within or near the upper limits or normal after recovery. Despite the unchanged promazine disposition, four out of six patients had more severe promazine side-effects, such as sedation, postural hypotension, and dizziness during the acute phase of the illness. This study suggests that promazine disposition was not significantly altered as a consequence of viral hepatitis. However, the pharmacodynamic effects of promazine were changed significantly. Care must be taken with patients who are taking promazine during the acute phase of viral hepatitis B.