A chip-based P450 in vitro metabolism assay coupled with ESI-MS and ESI-MS/MS detection is described in this paper. The chips were made of a cyclic olefin polymer using a hot embossing process. The introduction of reagent solutions into the chip was carried out using fused-silica capillaries coupled to two syringes with the flow rate controlled by a syringe pump. Initial experiments described here employed a small commercial guard column in an off-chip format to desalt and concentrate the products of the enzymatic reaction prior to ESI-MS analysis. The system was used both to yield the Michaelis constant (K(m)) of the P450 biotransformation of imipramine into desipramine and to determine the IC50 value of a chemical inhibitor (tranylcypromine) for this CYP2C19-mediated reaction. The results demonstrated that the kinetics of the reaction inside the 4-microL volume within the channels of the cyclic olefin polymer chip provided results in agreement with those reported in the literature using conventional assays. The above reactions were carried out using human liver microsomes, and the metabolites were detected by ESI-MS showing the potential of the chip-based P450 reaction for metabolite screening studies as well as for P450 inhibition assays. A porous monolithic column was subsequently integrated into the chip to perform the reaction mixture cleanup process in an integrated fashion on the chip that is necessary for ESI-MS detection. The miniature monolithic SPE column was prepared in situ inside the chip via UV-initiated polymerization. The results obtained using the integrated system demonstrated the possibility of performing P450 enzymatic reactions in a microvolume reaction chamber coupled directly to ESI-MS detection and required less than 4 microg of HLM protein.