Changes in handling of Li⁺ by contralateral kidney during acute Li⁺ loading were investigated immediately after unilateral ureteral obstruction. Carotid artery, jugular vein, renal vein and ureter of experimental animal were catheterized and renal venous flow was shunted to .external jugular vein. In experimental group right ureter was ligated. One to two hours after operation a single shot of LiCl solution (2 mEq/kg) was intravenously injected and then .arterial, renal venous blood and urine samples were taken sequentially for 1 to 1½ hours. Urine volume, plasma and urinary concentrations of Li⁺, Na⁺ and K⁺ were measured and urinary excretion of them were calculated. Results obtained were as follows: 1) In experimental group urine volume, urinary excretion of Na⁺, and K⁺ by contralateral kidney after unilateral ureteral obstruction were slightly larger than mean value of both kidney in control group. 2) During acute Li⁺ loading contralateral kidney in experimental group showed limited K⁺ excretion, but urinary flow and Na⁺ excretion were comparable to mean value of both kidney in control group. 3) Urinary osmolar concentration in experimental group was much lower than that in control group, and it was maintained at low level even after Li loading. 4) In experimental group plasmaLi⁺ concentration decreased more slowly than in control group after a single shot of LiCl solution. 5) Urinary excretion of Li⁺ in experimental group was markedly decreased, even lesseer than mean of both kidney in control group. 6) From the above results it was concluded that immediately after unilateral ureteral obstruction contralateral kidney showed normal water and Na⁺ diuretic response to Li load but urinay Li⁺ excretion was decreased and reclaimed Li⁺ to systemic circulation.