Plasma chlorpromazine (CPZ) levels were studied in children 8 to 15 yr of age who were receiving CPZ for mental disorders. The results showed that children need larger doses of CPZ than adults to attain comparable plasma concentrations. The apparent therapeutic plasma concentration in children (40 to 80 ng/ml) is lower than that reported for adults (50 to 300 ng/ml). In the children studied, the plasma concentrations declined despite maintained dosage, possibly because of autoinduction of CPZ metabolism. Doses of 6 to all mg/kg body weight, and even as much as 850 mg/day, were tolerated without significant side effects when administered in divided doses 3 or 4 times a day, while doses of less than 6 mg/kg/day frequently failed to produce the desired plasma CPZ levels.