Incorporation of 15N into blood plasma proteins and blood cells was studied in 11 patients with ischemic heart disease and peripheric atherosclerosis and in 7 healthy volunteers after oral administration of 15NH4Cl (95 atm%/15N) at a dose of 3 g per 70 kg of body mass. Metabolism of fibrinogen was stimulated, while incorporation of 15N into thrombocyte proteins was decreased in the patients with atherosclerosis as compared with healthy persons. Possible reasons of these alterations are discussed.