The quaternary ammonium-linked glucuronides of tripelennamine and cyproheptadine, important human metabolites of these drugs, were synthesized with an immobilized rabbit hepatic microsomal enzyme system as catalyst. The glucuronide of pyridine was prepared by chemical synthesis. These cationic, thermally labile, quaternary ammonium-linked glucuronides were analyzed by fast-atom bombardment mass spectrometry. The mass spectra contained intense molecular cations and characteristic losses of neutral fragments. The information contained in the spectra was sufficient to characterize the difficult-to-analyze intact quaternary ammonium glucuronides and provide structural information about the aglycon.