[6-14C,2-3H]Nicotinic acid, [6-14C,6-3H]nicotinic acid, [methyl-14C,2-2H,3H]trigonelline, and [methyl-14C,6-2H,3H] trigonelline were all incorporated into the isoquinuclidine moiety of the alkaloid dioscorine found in the yam Dioscorea hispida with complete retention of 3H relative to 14C. A chemical degradation] on the dioscorine derived from the labelled trigonellines indicated that all the 14C was located on its N-methyl group. 2H NMR spectroscopy established that the samples of dioscorine derived from the [2-2H]- and [6-2H]trigonelline were labelled with deuterium at the 3-pro-R and C-1 positions, respectively. These results are consistent with a new hypothesis for the biosynthesis of dioscorine. © 1988.