Sparteine, an antiarrhythmic and oxytocic drug, is metabolised by N1-oxidation. The sparteine-N1-oxide rearranges with loss of water to 2- and 5-dehydrosparteine. 18 (i.e., 5%) out of 360 subjects were unable to metabolise the drug. These persons, who were designated as nonmetabolisers, excreted almost 100% of the administered dose in urine as unchanged drug. The defective metabolism of sparteine was found to have a genetic basis. Sparteine-N1-oxidation appears to be determined by two allelic genes at a single locus where nonmetabolisers are homozygous for an autosomal recessive gene.