Lysine biosynthesis in seedlings of barley (Hordeum vulgare L. var. Emir) was studied by direct injection of the following precursors into the endosperm of the seedlings: acetate-1-(14)C; acetate-2-(14)C; pyruvate-1-(14)C; pyruvate-2-(14)C; pyruvate-3-(14)C; alanine-1-(14)C; aspartic acid-1-(14)C; aspartic acid-2-(14)C; aspartic acid-3-(14)C; aspartic acid-4-(14)C; alpha-aminoadipic acid-1-(14)C; and alpha, epsilon-diaminopimelic acid-1-(7)-(14)C. The distribution of activity in the individual carbon atoms of lysine in the different biosynthetic experiments was determined by chemical degradation. The incorporation percentages and labeling patterns obtained are in agreement with the occurrence of the diaminopimelic acid pathway. The results do not fit the incorporation percentages and labeling patterns expected if the alpha-aminoadipic acid pathway was operating. However, the results show that barley seedlings are able to convert a small part of the alpha-aminoadipic acid administered directly to lysine.The labeling pattern of lysine was found to be symmetrical around carbon 4. This indicates that the biosynthetic pathway proceeds via a symmetrical intermediate like ll-alpha, epsilon-diaminopimelic acid, or includes compounds as 2, 3-dihydrodipicolinic acid or Delta(1)-piperideine-2, 6-dicarboxylic acid which probably isomerise with concomitant lack of asymmetry in the labeling. The percentages of incorporation show that both the mesoand ll-forms of alpha, epsilon-diaminopimelic acid are metabolically convertible to lysine in seedlings of barley.