To evaluate a critical concentration concept of cadmium (Cd) toxicity on the kidney, relationships of renal Cd level with urinary excretion of various substances—i.e., me‐tallothionein, alkaline phosphatase, lactate dehydrogenase, N‐acefyl‐β‐D‐glucosa‐minidase, total protein, Cd, copper, and zinc—were studied in Cd‐injected rats. At the renal Cd concentration of 100–200 μg/g tissue, a dramatic increase of all these substances in urine was observed, supporting the idea of the critical concentration proposed by Friberg and co‐workers (1974). The significance of increase of urinary metallothionein below this level is also discussed.