The simultaneous quantitative assay of 1,4-dihydropyridine calcium antagonist manidipine and its main photodegradation by-product has been defined by using a multivariate calibration on UV spectra based on a classical least squares regression. Optimization of the procedure was achieved by means of a calibration model using suitable wavelength ranges singled out from a fractionation scheme thereat defined. Recovery values of 99 and 96% for the drug and the by-product, respectively, were found either in appropriately prepared mixtures and commercial formulations. Quantification limit for the photoproduct concentration was estimated as 1.0% for the reference samples.