Doubly-labeled [(3)H, (14)C]tyrosines, [1-(13)C-]tyramine or [2-(14)C]tyramine, administered to the stems of intact Papaver somniferum L. plants, were found to be incorporated into the morphinan alkaloids of the plant with comparable efficiency. (3)H/(14)C ratios of alkaloids from plants fed the tyrosines were consistent with an almost equal conversion of this amino acid into the tetrahydroisoquinoline (TIQ) and benzyl-derived segments. Nuclear magnetic resonance (NMR) analyses of morphine isolated after administration of [1-(13)C]tyramine demonstrated selective labeling of C-16 of the alkaloid, indicating the conversion of this amine primarily into the TIQ-derived moiety. Morphine and thebaine labeled by [2-(14)C]tyramine were degraded to phenanthridines and N,N'-dimethyl ethylamines. Of the total radioactivity in the alkaloids 97% was found to be associated with the ethylamines, a distribution consistent with the NMR data. This preferential utilization of tyramine in the biosynthesis of morphinan alkaloids can be explained by the compartmentalization of intermediates and enzymes of the pathway.