Routine use of the USP XXI spectrophotometric method for the content uniformity determination of hydralazine hydrochloride tablets has shown that tablet excipients can significantly alter the spectral characteristics of the drug and thus cause inaccurate assay values to be obtained. Because of this problem, a simple and reliable alternative spectrophotometric assay method, based on the conversion of hydralazine to tetrazolo [5,1-alpha]phthalazine with nitrite ions under acidic conditions, was developed. The derivative showed an absorption maximum at about 274 nm and obeyed Beer's law over the concentration range 4-40 micrograms/mL. Mean recoveries of hydralazine hydrochloride added to commercial coated and uncoated tablets were 101.0% (n = 10) and 100.8% (n = 8), respectively. The proposed method was found suitable for the assay not only of individual tablets but also of tablet composites.