After administering radio-isotope Hg(203) Cl(2) into guinea pigs the author estimated the quantity of radio-isotope mercury excreted inthe urine and feces. At the same time the author studied the effect of Ca-EDTA administration on the excretion of Hg(203) Cl(2) and also the mercury distribution in the principal organs of the aminals, and arrived at the following conclusions. 1. During the period one to four days after the Hg(203) Cl(2)-adminstration the quantity of mercury excreted in the urine increases markedly, and thereafter it tends to decrease rather abruptly. On the other hang, the amount of mercury excreted in feces is greatest during the period from the third to fifth day after the administration, and it tends to decrease somewhat slowly thereafter. The amount of mercury rexcreted in the urine is as much as twice or 2.7 times that excreted in the feces. 2. One week after the administration of Hg(203) Cl(2) a considerably large amount fo it is still adsorbed in the kidneys, but it is detected only in a small amount in the organs such as the liver and caecum, and likewise the quantity detected in other organs is extremely small. 3. The administration of Ca-EDTA tends to increase markedly the amount of Hg(203) Cl(2) excreted in the urine. 4. However, no markedly increasing effect of the Ca-EDTA administration can be recognized on the excretion of Hg(203) Cl(2) in the feces. 5. The amount of Hg(203) Cl(2) adsorbed in the principal organs tends to decrease slightly in the kidneys and liver after the adminstration of Ca-EDTA.