To treat plutonium poisoning, the injn. soln. contained 20-25 mg. of Zr in 10% Na citrate. Young adult $ rats and a young mongrel dog were used. The rats received intraperit. and the dog intraven. injns. Zirconium treatment in rats produced as much as a 15-fold increase in the amt. of plutonium excreted in the urine over a 24-hr. period. When the output of plutonium had dropped, reinjection of Zr caused a second rise in the elimination rate. The excretion level remained about 3 times greater than that of the controls for several days after the cessation of therapy. None of the treatments altered the amt. of plutonium in the feces. The treated rats had only about half as much plutonium in the skeleton and in the liver as the controls, but the concn. in the muscle and kidneys was greater than that in the controls. Five months after the dog received an intraven. injn. of 0. 058 mg. /kg. of plutonium, treatment with 4-16 mg. /kg. of body wt. of Zr, intraven., was begun. A definite increase in urinary excretion of plutonium occurred within the 1st few days following each dose of Zr. The rise in elimination rate varied directly with the amt. of Zr injd. The increased rate of excretion was continued for at least 3 wks. after stopping Zr. The elimination rate of urinary plutonium was increased more than 150% over the pretreatment level. The excretion in the feces increased only slightly. In rats treated with Zr, 5 wks. after injn. of plutonium, a 10-fold increase in urinary excretion of plutonium occurred. At autopsy these animals had only 1/2 as much plutonium in the liver as the controls. The bone content was unchanged. It appears that Zr first displaces plutonium from the liver. Later it migrates to the bone and displaces the plutonium to an extent which depends on the concn. of Zr relative to the plutonium. Zr was found to be the safest metal for therapy. When given in single massive doses of 1. 5, 1. 75, and 2. 0 g. /kg. of body weight, the percentages surviving were 100, 50, and 0, resp. There. were no harmful chronic effects. Within 24 hrs. after injn., 80-90% of Zr was excreted in the urine. Its fecal excretion was very low. Na citrate promotes excretion of plutonium by increasing the diffusion of plutonium and thus facilitating its clearance through the kidney.