The metabolic pathway of dimetindene (dimetindene maleate, Fenistil), an antiallergic drug commercially available for more than 20 years, has remained unknown. By means of HPLC, GC-MS and MS-MS, dimetindene was found to be hydroxylated on the indene moiety of the molecule both in vitro with hepatic microsomes of several species and in vivo in man. Cochromatographic analysis (HPLC and GC-MS) of the primary metabolites produced in vitro and in vivo demonstrated their similarities but revealed differences from the chemically synthesized 5-hydroxy-dimetindene. Using large volumes of in vitro incubations of rat microsomes with dimetindene and cofactors, 1.15 mg of pure primary metabolite were first produced and then extensively purified. The study of the NMR proton in one and two dimensions (COSY) clearly demonstrated that this metabolite is hydroxylated on the C6 of the indene moiety as compared to the C5 for the synthetic product.