A simple, rapid and high selective method for the assay of captopril has been developed. It is based on the reaction of captopril with Cu2 and SCN-to form precipitate quantitatively in aqueous solution. The amount of residual Cu2 + can he determined by flow injection on-line filtration-dilution-atomic absorption spectrometry, then the content of captopril can be determined indirectly. The linear range is 2-100 mg/L, the recovery is 97. 1 %-99.5%, the sampling frequency is 100/h. The method has been applied ...