An in vitro technique for the determination of radiopharmaceutical metabolism has been developed using isolated hepatocytes. Radioiodinated N-isopropyl-p-iodoamphetamine (IMP; iofetamine, USP) was employed a model tracer in these studies because its labeled metabolites are well-characterized. Hepatocytes isolated from the rat and human produced labeled metabolites in vitro for up to 4h in a manner similar to that reported for humans in vivo. Identical metabolites were generated by all cell types investigated, but the rate of metabolism differed (rat ≫ human; female > male; fresh > frozen). The utility of this methodology for the preclinical evaluation of potential radiopharmaceuticals is described.