Chemical changes of proteins caused by gamma irradiation usually comprise fragmentation, cross-linking and oxidation by free radicals generated in the radiolysis of water. The last process is thought to play a major role in many oxidative processes of proteins within cells and is implicated in a number of human diseases as well as ageing. Most of already reported proteins modification mediated by ionizing radiation with the corresponding side products formed have been already reviewed [Chem Rev, (1987), 87, 381]. In order to initiate a study aiming at examining the radiation effect upon peptide structures, the present work selected the angiotensin II (AII, MW 1046 Da) as model. The Asp-Arg-Val-Tyr-Ile-His-Pro-Phe sequence was submitted to different doses of gamma radiation (1, 2, 4, 6, 8, 10 and 15 kGy) in aqueous solution and in presence of oxygen. Biological assays of these treated peptides after irradiation at 4, 6 and 10 kGy revealed that their potencies decreased to 25, 8 and 3.5 % in rat uterus and 25, 12 and 3% in pig ileum, respectively. These findings stem for a degradation process that occur in the peptide when submitted to radiation. In accordance with these biological data, the HPLC and mass spectroscopy monitoring of these treatments indicated that the higher the irradiation dose the greater the amount of peptide products formed with different polarities and molecular masses. Amongst the two main products generated that seem to be dose-dependent, a component with +16 Da appeared at 1 kGy and increased until 8 kGy but decrease at 10 kGy. This first product seems to be resultant of Phe oxidation by OH• radical, leading to the formation of m- and o-tyrosine [Amino Acids (2003), 25, 207]. The second side product which appeared at doses higher than 4 kGy is probably derived from the Phe residue oxidation, initially producing the o- or m-tyrosine, subsequently undergoing a new oxidation step with the production of the 2,3 - dihydroxyphenylalanine (DOPA) or 2,5 DOPA compounds (+ 32 Da). In addition, there might occur the oxidation of His leading to Asn (-23 Da), Pro leading to pyroglutamic acid (+14 Da) and Tyr leading to 3,4 DOPA (+16 Da) totalizing the increase in 39 Da. Collectively, the results suggest that the radiation effect on AII seem to be sequence-dependent as the amino acid residues located mainly at the C-terminal position are more prone to modification than those at the N- terminal region of the peptide. Supported by Fapesp and CNPq.