Weanling rats were maintained on diets which were Mg-adequate (49 mmol Mg/kg), groups 1 and 3, or Mg-deficient (8 mmol Mg/kg), groups 2 and 4, for 8 weeks of growth. During breeding, gestation, and lactation, groups 3 and 4 received diets supplemented with 41 mmol Mg/kg. Femur and cardiac Mg in group 2 dams was reduced, confirming that Mg deficiency was present. Thymic weight from dams and pups and numbers of plaque-forming cells in pups in group 1 were higher than in other groups while splenic weight in pups was lowest in group 2. Results indicate that maternal magnesium deficiency impaired immunocompetence in offspring. Regardless of the presence or absence of pregestational magnesium deficiency, magnesium supplementation also was associated with depressed immune function.