To localize the metabolic block(s) in the biosynthesis of glucosinolates in the Brassica napus L. cv. Bronowski, 5-methylthiopentanal oxime-1-¹⁴C was synthesized and fed to this cultivar and cv. Regina II, which has average levels of glucosinolates. The aldoxime was as efficient as a precursor of 3-butenylglucosinolate in both cultivars, but less efficient as a precursor of 2-hydroxy-3-butenylglucosinolate in Bronowski. These results suggest that there is a block in the biosynthesis of 3-butenylglucosinolate in Bronowski that is situated before the synthesis of the intermediate 5-methylthiopentanal oxime, as well as a block in the hydroxylation step. The silique walls of Bronowski contained increased amounts of total sulphur and inorganic sulphate.