Continuing earlier experiments in which deoxyloganic acid proved to be a precursor of the doubly decarboxylated iridoids in Thunbergia alata, we have now shown that deuterium labelled 6-deoxyretzioside was incorporated into stilbericoside in this plant. In the same experiment labelled retzioside was isolated, indicating that this is also a likely intermediate in the biosynthetic pathway. Feedings in two species of Deutzia were also performed with deuterium-labelled iridoids, namely iridodidal glucoside, 8-epiiridodial glucoside, 7-hydroxy-iridodial glucoside and 10-hydroxy-iridodial glucoside (=decapetaloside). Of these, only the first and the last were incorporated. Apparently, loss of C-10 in the biosynthesis of scabroside is analogous to that of stilbericoside and the sequence of events after formation of the 7,8-double bond appears in both cases to be the same as that found for the biosynthesis of antirrhinoside. The close relationship between Nuxia, Retzia and the Stilbaceae indicated by chloroplast DNA sequencing results is substantiated by the common occurrence of a special kind of iridoids in these taxa. A similar close relationship is found between Hydrangeaceae and Loasaceae. (C) 1999 Elsevier Science Ltd. All rights reserved.