Datura stramonium L. plants were allowed to metabolize certain C14-labeled compounds for 10 days following which hyoscyamine and scopolamine were isolated and purified. Of the compounds employed, phenylalanine-3-C14, phenylacetic acid-1-C14, and sodium acetate-2-C14 were the most efficient precursors, followed by sodium propionate-2-C14, sodium formate-C14, and serine-3-C14 in that order; zinc lactate-2-C14 proved to be a comparatively inefficient precursor. The distribution of C14 within the alkaloids was entirely in their tropic acid moiety when phenylalanine and phenylacetic acid were used as precursors, and almost entirely in their tropine or scopine portions when acetate, propionate, formate, and serine were employed. The samples of tropic acid obtained, following administration of phenylalanine and phenylacetic acid, were degraded and the location of the C14 determined.