A radioimmunoassy (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-succinoylpropylbupropion 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 wiht 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 2 beagles of each sex after i.v. and oral administrations of bupropion hydrochloride (100 mg). The pharmacokinetic profile in dogs was best described by an open 2-compartment model after either route of drug administration. Peak plasma bupropion levels after oral dosing were highly variable, 12.9-63.5 ng ml-1 at 26-32 min after drug administration. The mean terminal phase half-life of bupropion was calculated to be 1.73 h after either route and the absolute oral bioavailability of the drug varied at 2.0-6.5%.