The effect of tin on the calcium binding activity in renal cortex was studied in rats administered stannous chloride intraperitoneally. The calcium concentration in the soluble fraction of renal cortex was elevated about 30‐fold over that in controls 2 days after administration of tin (3.0 mg/100 g). The calcium binding activity in the soluble fraction of renal cortex increased significantly at any of the time intervals between 6 and 72 hr after administration of tin, and this increase preceded an elevation of the calcium concentration in the renal cortex. The increased calcium binding activity produced by administration of tin was strikingly reduced by treatment with actinomycin D (100 μg/100 g). A significant increase in calcium binding activity was not exhibited after administration of zinc, manganese, calcium, or lead. These results suggest that the calcium accumulation in the renal cortex caused by administration of tin is closely connected with an increase in calcium binding activity.