The biosynthesis of the branched fatty acid moiety of Capsaicinoids was examined by radioisotopic technique. Labelled nordihydrocapsaicin (NDHC), homocapsaicin I (HC I) and homodihydrocapsaicin I (HDHC I) were isolated from CAPSICUM fruits which had been fed L-leucine-U- (14)C via cotton wicks inserted into the stems. After the administration of L-i-leucine-U- (14)C radioactivity could be detected in the HC II- and HDHC II-fractions. As L-valine serves as precursor of L-leucine, not only labelled capsaicin (C) and its dihydroderivative (DHC) resulted when L-valine-U- (14)C was fed, but also NDHC, HC I and HDHC I. The results demonstrate that valine, i-leucine and leucine are direct precursors of the respective even- and odd-numbered branched fatty acid moieties of the capsaicinoids.