Para-iodoamphetamines are currently used in nuclear medicine to detect brain perfusion abnormalities with tomoscintigraphy. Little is known about their metabolism pathways in rat and humans. N-isopropyl-125I-iodoamphetamine (IMP) interactions were studied with rat liver microsomes. The first dealkylated metabolite (IAMP) at low concentration gave a type I binding complex then, with a high concentration, a very stable type II complex with oxidized cytochrome P-450 FeIII. In contrast, IMP only gave a type I binding complex in the absence of NADPH. In the presence of NADPH, IAMP and IMP produced 455 nm absorbing complexes, which were enhanced when phenobarbital-treated rat liver microsomes were used. During in vitro metabolic activation, covalent binding of IMP and IAMP on rat liver microsomal proteins was observed. This process was mixed function oxidase (MFO) dependent. The covalent binding level was higher with IAMP and was not affected by flavine oxidase inhibitors. These results confirm the interaction of IMP and IAMP with microsome proteins and cytochrome P-450 and suggest that an N-oxidation of IMP occurs after N-dealkylation. As cytochrome P-450 and dealkylated IMP (IAMP) were found in brain, cerebral metabolism in brain and evolution of activity biodistribution with the course of time can be suggested.