The effects of hydroxyl radical attack on a peptidic drug were studied in vitro. Different chemico-physical techniques were used to investigate structural damage induced by oxidative stress conditions in salmon calcitonin (sCT), a peptide hormone used in treating osteoporosis. Reversed-phase liquid chromatography (RP-HPLC), circular dichroism (CD) and transmission electron microscopy (TEM) were applied to measure formation of oxidation/degradation products and to reveal the conformational and ultrastructural modifications in the presence of OH. free radicals. Hydroxyl radicals were obtained from ferrous sulfate and ascorbic acid mixtures. The RP-HPLC results revealed the formation of new chromatographic peaks indicating a number of degradation/oxidation products formed in the presence of OH. free radicals. CD spectra showed slight protein conformational modifications as well as aggregation. TEM confirmed sCT aggregation and suggested the formation of fibrillar aggregates.