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Sparteine is reportedly metabolized in mammals with the formation of an N-oxide which undergoes dehydration to delta 2 and delta 5-dehydrosparteine. In our studies male Sprague-Dawley rats were found to metabolize sparteine and alpha-isosparteine to lupanine and alpha-isolupanine respectively in vivo. Metabolic conversion of sparteine in vitro in the presence of microsomal and 9000 x g supernatant fractions of the rat liver homogenate did not produce detectable lupanine. The in vivo studies were conducted by pretreating rats with inducers and inhibitors of microsomal enzymes. Inducers did not increase levels of lupanine in the rat urine but a significant decrease was observed in the presence of the inhibitor SFK 525A. Disulfiram reduced lupanine levels in the urine. The bioconversion of sparteine to lupanine appears to be mediated by microsomal enzymes and may proceed via an aldehyde intermediate. The conversion of sparteine to lupanine may parallel the mammalian metabolism of nicotine to cotinine.