Our previous investigation of the naturally occurring indole compounds in barley and tomato shoots suggested that the biosynthesis of indol-3yl-acetic acid (IAA) from tryptophan might proceed via either the indol-3yl-pyruvic acid or tryptamine pathways in both species. The results further indicated that the indol-3yl-lactic acid pathway for IAA formation might also be operative in tomato. In the present study, tryptophan-3-14C and tryptamine-2-14C were fed to excised shoots of both barley and tomato, and indol-3yl-lactic acid-3-14C was also fed to shoots of tomato. All three compounds were found to give rise to radioactive IAA with little dilution in specific activity. Feeding tryptophan-3-14C also resulted in the labelling of indol-3yl-pyruvic acid, indol-3yl-acetaldehyde, and tryptamine, which were isolated and chemically identified from both species, and radioactive indol-3yl-lactic acid and tryptophol were also produced in tomato. Indol-3yl-acetaldehyde was found to be labelled in both species after administration of tryptamine-2-14C, while the principal metabolite of indol-3yl-lactic acid-3-14C was radioactive tryptophan. These findings, along with the results from a quantitative study of the radioactive metabolites, indicate that both the indol-3yl-pyruvic acid and tryptamine pathways can operate in both species, while the formation of IAA from indol-3yl-lactic acid in tomato probably occurs indirectly, via tryptophan. These conclusions were supported by the demonstration of the enzymes, L-tryptophan transaminase, L-trypto-phan decarboxylase, and indol-3yl-acetaldehyde dehydrogenase in cell-free extracts of both tissues, and of indol-3yl-pyruvic acid decarboxylase in the tomato extract. No indol-3yl-lactic acid decarboxylase activity was observed in the extracts from either tissue.