Abstract.Angiotensin is readily inactivated by ultraviolet light presumably due to the photosensitivity of its tyrosyl and phenylalanyl residues. Alkalinity promotes and acidity delays the biological decomposition. Under conditions of oxygen exclusion (i.e. under N2or H2) the rate of decomposition is further depressed and when irradiation is carried out under hydrogen a biologically active product is formed. This altered angiotensin, judging from its chromatographic migration and dialysis rate, represents a polymerized form of angiotensin. Under identical irradiation conditions, angiotensin was coupled with C14labelled pheny‐lalanine which indicates that this residue might be the photoreactive species. The production of altered forms of angiotensin by ultraviolet irradiation was discussed with respect to possible utility in anti‐metabolite, antigen or tracer w