Benzylglucosinolate accumulates in mature plants of Tropaeolum majus L. The biosynthetic capacity for synthesis of benzylglucosinolate and the total content of benzylglucosinolate have been investigated during plant development and in different tissues. The content increased from 5 mg of benzylglucosinolate in the fresh seed to between 200 and 400 mg in the adult plant, depending on size. The biosynthetic capacity was measured using L-[U-14C]phenylalanine as precursor. Incorporation levels of approximately 30% were obtained with green leaves, whereas the incorporation levels obtained with other tissues were in the range of 0 to 5%. Leaves were the primary site of benzylglucosinolate synthesis. The high amounts of benzylglucosinolate accumulated in other tissues (e.g. developing seeds) reflected transport of benzylglucosinolate from the leaves. The initial steps in the biosynthesis of glucosinolates and cyanogenic glycosides are thought to be similar and to be localized on microsomal membranes. However, a microsomal system prepared from T. majus was biosynthetically inactive. Inclusion of T. majus plant material during preparation of sorghum microsomes also inhibited their activity. Benzylisothiocyanate, generated by degradation of benzylglucosinolate during the homogenization procedure, strongly inhibited the sorghum enzyme system, and its presence may thus explain why the isolated T. majus microsomal system is inactive.