β-cyanoalanine synthese in etolated seedlings of Lupinus angustifolius is found mainly in the mitochondrial fraction of the cotyledons and stems. In seedlings developing at 25°C in the dark it reaches a maximal concentration after 5 days, at the same time as asparagine accumulation is most rapid and other physiological changes occur. However, maximal abiliaty to assimilate HCN gas to asparagine develops after 3 to 4 days, before asparagine accumulation begins. The possibility that the β-cyanoalanine pathway is a major route for asparagine biosynthesis in lupin seedlings is concluded to be unlikely.