Ultra-performance liquid chromatography (UPLC) utilizes sub-2 μm particles with high linear solvent velocities to effect dramatic increases in resolution, sensitivity and speed of analysis. The reduction in particle size to below 2 μm requires instrumentation that can operate at pressures in the 6000–15 000 psi range. The typical peak widths generated by the UPLC system are in the order of 1–2 s for a 10-min separation. In the present work this technology has been applied to the study of in vivo drug metabolism, in particular the analysis of drug metabolites in bile. The reduction in peak width significantly increases analytical sensitivity by three- to five-fold, and the reduction in peak width, and concomitant increase in peak capacity, significantly reduces spectral overlap resulting in superior spectral quality in both MS and MS/MS modes. The application of UPLC/MS resulted in the detection of additional drug metabolites, superior separation and improved spectral quality. Copyright © 2004 John Wiley & Sons, Ltd.