When phenylalanine-2-C14was fed to sprouting barley the hordenine isolated from the plant was radioactive. The activity in the hordenine was shown by degradation of the alkaloid to be located on the carbon of the side chain β to the aromatic ring. A fraction of the fatty acids present in the roots was more strongly radioactive than the hordenine and must have arisen by the breakdown of the amino acid. The gramine isolated from the shoots was inert. Phenylalanine does not take part in the synthesis of gramine, but it is a precursor of hordenine and, hence, must be converted to tyrosine in barley. This conclusion is supported by the simultaneous synthesis of a radioactive aliphatic acid.