Chlorpromazine hydrochloride reacts with cerium ammonium nitrate to give a pink colour having maximum absorbance at 530 nm. The reaction is selective for chlorpromazine, with 0.001 mg/ml as visual limit of quantitation, and provides a basis for a new spectrophotometric determination. The colour reaction obeys Beer's Law from 0.001 mg to 0.5 mg/ml of chlorpromazine. The standard deviation does not exceed 0.050. The method was successfully applied to pure and pharmaceutical dosage forms of chlorpromazine. The quantitative assessment of tolerable limits of other drugs was also studied.