Male Wistar rats at 2, 4, and 8 months were given s.c. injections of CdCl2 at doses of 0.5, 1, and 2 mg Cd/kg, 3 days/week, for 4 weeks. Dose-related adverse effects observed at the end of the injections were as follows: decrease in body weight gain, increases in liver, kidney, and spleen weights, decrease in red blood cell counts, and increase in white blood cell counts accountable by an increased percentage of neutrophils in peripheral white blood cells. Essentially, all of these changes were age-related, i.e., the extents of the effects in each age group were in the order of 8- > 4- > 2-month-old rats when compared at the same dose level of Cd. The sensitive indicators of histological changes were increase in hematopoietic cells in bone marrow and fibrous tissue proliferation in the thymus > fibrous tissue proliferation and hyperplasia of lymph follicle in the spleen > renal tubular degeneration. These histological changes became to be marked at lower doses with aging. Dose-dependent increases in total Cd concentrations in the liver, kidney, and spleen were slightly higher with aging, while metallothionein (MT) contents in these organs were induced exactly in the same pattern as Cd concentrations in each organ in various age groups. This study revealed that the adverse effects after repeated administration of Cd on the kidney, spleen, thymus, and bone marrow worsened with increasing animal age and that this phenomenon could not be explained simply by differences in Cd disposition or MT induction in the organs of the different age groups.