A simple difference spectrophotometric method has been developed for the selective and rapid determination of minoxidil in pharmaceutical formulations. The method is based on the characteristic modifications of the minoxidil UV spectrum induced by pH changes; a difference spectrum can be obtained and the absorbance A282.8 or the absorbance difference delta A = (A282.8-A259.2) can be used for the selective drug quantitation. As an alternative, a derivative second-order spectrophotometric method was also developed. The spectrophotometric methods were applied to the quality control of commercial minoxidil dosage forms and the results were comparable with those obtained by a reference HPLC procedure.