Chlorprothixene (CPX) and CPX sulphoxide were demonstrated in breast milk from two psychotic mothers taking 200 mg CPX daily. The milk concentrations of CPX were 120 to 260% greater than in plasma. The estimated amounts of drug administered in breast milk to one of the infants were 15 and 26 µg/day for CPX and CPX sulphoxide, respectively. Accordingly, the infant dose of the parent compound would be only 0.1% of the maternal dose/kg body weight. It is not likely that CPX or its metabolite would exert any immediate pharmacological effects in the nursing infant. However, the long term effect of low doses of neuroleptic drugs in the developing infants is not yet known.