Capsaicin, the crystalline pungent principle of Capsicum annuum, has been shown to be a mixture of at least five closely related vanillyl amides. Phenylalanine, various hydroxylated cinnamic acids, and vanillylamine, all labelled with tritium in the aromatic ring, were incorporated biosynthetically into the C6–C1 unit of capsaicin. Tyrosine was incorporated much less efficiently than phenylalanine. The radioactive capsaicin mixture derived biosynthetically from phenylalanine was found to contain most of the activity in the minor, saturated components.