The absorption, deposition, and excretion of zinc65 was studied in mice, rats, and dogs. When zinc65 was fed to the animals it was poorly absorbed, but its long biologic half-life made even the small portion absorbed physiologically significant. Absorption was obviated by feeding large quantities of nonradioactive carrier zinc. Injected zinc65 chloride first was deposited, preferably in the pancreas, liver, and spleen, with only minor deposition in muscle and in the brain. Subsequently, a large proportion was transferred to bone. The chief means of excretion was in feces, presumably via pancreatic secretion. Injected nonradioactive zinc or treatment with 2,3-dimercaptopropanol (BAL), Versene, or cadmium ion failed to alter body burden significantly. Cadmium, however, decreased soft tissue zinc65 deposition and increased accretion by the skeleton. Blood activity fell rapidly and less than .01% of the dose injected remained in the blood after 1 day. Renal excretion of zinc65 rose as plasma concentration fell and clearance ratios in excess of 1.0 were noted, indicating probable renal secretion.