A radioimmunoassay (RIA) procedure for the quantification of bupropion (dl-2-tert-butylamino-3'-chloropropiophenone) in biological fluids is described. Immunization of rabbits with conjugates of bovine serum albumin and p-succinoyl propylbupropion or p-carbomethoxybupropion resulted in the production of antisera which are capable of detecting less than 1 ng ml-1 (100 pg actual mass) of bupropion in the RIA, utilizing [6-3H] bupropion as radioligand. The antisera used in these studies have low cross-reaction (approximately 0.1% or less) with known side chain metabolites of bupropion, but exhibit significant cross-reaction with p-hydroxybupropion (30.3%). Excellent agreement was obtained between RIA and high-pressure liquid chromatography determinations of bupropion concentrations in human plasma samples, but plasma or serum from bupropion-treated dogs, rats and mice required extraction from basic medium to remove some interference before RIA. The assay was applied to a study of bupropion disposition in two beagles of each sex after i.v. and p.o. administrations of bupropion hydrochloride (100 mg). The pharmacokinetic profile in dogs was best described by an open two-compartment model after either route of drug administration. Peak plasma bupropion levels after oral dosing were highly variable, ranging from 12.9 to 63.5 ng ml-1 at 26 to 32 min after drug administration. The mean terminal phase half-life of bupropion was calculated to be 1.73 hr after either route and the absolute oral bioavailability of the drug varied from 2.0 to 6.5%.