Bupropion provided a dose-dependent prevention of tetrabenazine-induced sedation in mice but not rats. Bupropion was extensively metabolized in mice, rats, dogs and man. About 85% of the dose was excreted in urine of rats and man. The predominant metabolites in rat urine were side chain cleavage products of bupropion (m-chlorobenzoic acid) with a minor fraction consisting of basic side chain hydroxylated metabolites. Mice, dogs and man form a major side chain hydroxylated product (BW 306U) which appeared in higher concentration than bupropion in plasma of these species but not rats. The relatively high plasma levels of BW 306U in mice but not rats may account for the species difference in pharmacological response observed with bupropion.