After oral and intravenous administration of radiolabelled isobutylnaphthyl acetic acid (INAA) to rats two metabolites were isolated from urine and plasma by HPLC. Field desorption, high resolution electron impact mass spectrometry as well as GC-MS after derivatization were used for structure elucidation and identification of the metabolites. The main biotransformation product in rat urine was found to be 5-(2'-hydroxy-2'-methyl-propyl)-1-naphthyl acetic acid (M1). The main metabolite in plasma was derived and was found to be 5-(2'-carboxypropyl)-1-naphthyl acetic acid (M2).