Abstract Phenyl[1- 14C] lactic acid and phenyl [2- 3H] lactic acid were synthesized by conventional routes and fed in admixture ( 3H: 14C ratio 10:1) to Datura stramonium via the roots. Hyoscine (scopolamine) and hyoscyamine were isolated and found to have essentially the same 3H: 14C ratios as the precursor (8.5 and 10.15, respectively). Hydrolysis of hyoscyamine showed that all the activity was present in the tropic acid moiety. The tropic acid was converted to atropic acid and then cleaved by means of osmium tetroxide and sodium metaperiodate to yield formaldehyde containing all the tritium and phenylglyoxylic acid containing all the 14C, a result which shows that the tritium at C-2 of the phenyllactate precursor labelled tropic acid in the hydroxymethyl group. These findings show that phenyllactate is an obligatory intermediate in the biosynthesis of tropic acid and that the rearrangement of the side-chain takes place at the phenyllactate and not the phenylpyruvate level.