Cholesterol-C14 and cholesterol-H3 were fed to rabbits for 1 week and cholesterol was isolated from aortae, livers, small intestines, adrenals, kidneys, brains and blood. Isolated material from all tissues had a lower count than the material fed. It was surprising that cholesterol-H3 counts were always comparatively lower than cholesterol-C14 counts. This discrepancy may be explained by a peripheral oxidation of the cholesterol-H3 molecule, in which it loses radioactive hydrogen and therefore radioactivity. This peripheral oxidation is apparently mainly operative in blood. If after feeding of radioactive material non-radioactive cholesterol is fed for 7 or 11 weeks, radioactivity in both types of experiments is reduced to a small fraction of original values. From these data one can calculate that about 0.03 mg cholesterol are laid down in the aorta/animal/day/g of wet tissue, a very small part of the 900 mg fed/day/animal. The experiments show that there is a continuous exchange of radioactive cholesterol between tissues and blood in which even the plaques, formed in the aortae, must take part.