Cadmium-induced metallothionein (MT) synthesis was investigated in male rats of three ages, 3, 12 and 24 months. Physiological levels of MT in kidney, liver and lung measured simultaneously in untreated animals were found to be within the same range in all three age groups. Following 3-day treatment with CdCl2 equivalent to 1 mg/kg/day Cd, renal MT increased 10-15-fold in all animals. However, hepatic MT became elevated to 36 times the original value in the youngest (3-month-old) rats and 90 and 74 times, respectively, in the 12- and 24-month-old groups. Lung MT, which responded rather poorly to the cadmium inducer, increased 2-fold in the young group, but about 10 and 8 times, respectively, in the two older groups. High mortality of 75% occurred in the aged (24-month-old) group following cadmium administration, indicating age to be an important sensitizing factor in the toxic hazard of heavy metal exposure. The possible role in this connection of MT is discussed.