A simple and sensitive spectrophotometric method is described for the assay of some tranquillizers and antidepressants, namely, chlorpromazine, imipramine, amitriptyline and chlorprothexine. The method was based on the interaction of these drugs as n-electron donors with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as λ -acceptor to give a highly coloured radical anion which exhibits maximum absorption at 460 nm. The radical anion was proved by electron spin resonance measurements. The proposed method has been successfully applied for the determination of the examined drugs in tablets. The assay results were in accord with the pharmacopoeial results.