Rapid screening for and selection of potential metabolites of urapidil in dog urine by high-performance liquid chromatographic analysis is achieved by use of a diode array UV detector, which allows multi-wavelength detection and collects several complete UV spectra from each peak in the course of a single chromatographic analysis. Because of the similarity of metabolite spectra to the spectrum of the parent compound, few interesting peaks are picked out, rather rapidly, by means of a selection criterion based on absorption ratios. Inspection of the complete UV spectra of these pre-selected peaks allows further restriction to "candidate" metabolite peaks. Comparison of up-slope, apex and down-slope spectra is a convenient way of testing for peak purity.