The first exact studies on the radiation chemistry of the peptide bond were performed by Garrison et al. (1, 2), who irradiated N-acylamino acids (N-acetylglycine and N-acetylalanine). A reaction mechanism was suggested for the radiolysis of the peptide bond in oxygenated and oxygen-free solutions based on the yields and types of products observed. The fundamental reactions found by Garrison and Weeks in oxygenated solutions of acetyl. glycine were cleavage of the peptide bond under formation of the amide of the N-terminal and the carbonyl derivative of the C-terminal amino acid (the keto acid). In oxygen-free solutions they observed similar cleavage products of the peptide bond besides recombination on C-2, yielding diaminosuccinic acid and aspartic acid from irradiated acetyl glycine (recombination of the acetic and glycine radicals or of two glycine radicals), diaminosuccinic acid being the major product. Hatano (3) compared the G(NH3) yield from several peptides in oxygenated solutions (2 X 105 R) and found decreasing values with increasing chain length of the amino acid participating in the peptide bond (Gly Gly, G = 4.02; Ala Leu, G = 1.05). In irradiated alanyl valine solutions he found formation of a-ketoisovaleric acid as well as of pyruvic acid, contradicting Garrison's observations. Simultaneous and independent experiments were carried out in our laboratory with leucyl glycyl glycine, leucyl glycine, and glycyl leucine (4) and a series of other peptides based on our previous work on the radiation chemistry of higher aliphatic amino acids (5-10), which established a very detailed radiolytic mechanism of these compounds in oxygenated and oxygen-free solutions. Our present studies aimed at elucidation of the effects of the peptide bond on the radiolysis of the amino acids and vice versa, since the results could contribute information on the radiolytic reactions of proteins. In this paper the products of radiolysis of eleven dipeptides and tripeptides were determined before and after hydrolysis of the solutions irradiated in the presence and in the absence of oxygen. The results show that there is an essential difference in the radiation chemical reactions in oxygenated and in oxygen-free solutions.