A simple and rapid colorimetric method for the determination of isoniazid, iproniazid phosphate, phenylhydrazine hydrochloride and hydrazine hydrate is described. The method is based on the formation of a Schiff base when these compounds react with 4-dimethylaminocinnamaldehyde and measurement of the condensed products at 370–555nm. The procedure has been successfully applied to the assay of the pharmaceutical preparations containing the studied drugs and hydrazine, and phenylhydrazine mixture, and the results are compared to the official methods. All the different experimental conditions are studied and incorporated into the procedure.