The occurrence of y-hydroxyarginine in plants has been confirmed by the isolation of its lactone from seeds of Vicia sativa. A new natural amino acid, y-hydroxyornithine, has been identified by chromatography and ionophoresis in the seeds of Vicia onobrychoide8 and V. unijuga. The closely related genera, Lathyrus and Vicia, characteristically store different groups of guanidino amino acids. A colour reaction for the identification of y-hydroxyornithine on paper is described. The new amino acid y-hydroxyhomoarginine has been isolated from seeds of Lathyrus tingitanus, as its lactone, and its structure confirmed. The biosynthesis of y-hydroxyhomoarginine from L-homoarginine in the same species has been demonstrated. All species of Lathyrus known to contain high concentrations of lathyrine in their seeds also contain a compound identified by ionophoresis as y-hydroxyhomoarginine.