The cell proliferation of bone marrow cells in rats, suffering from severe malnutrition during the lactation period, was investigated by using bromodeoxyuridine (BdUr) mixed with 100 mg of activated charcoal injected i.p. The undernourished rats had significantly more mitosis in the first generation (82.6%) than did well-nourished rats (44.6%) at 18 hours after the BdUr injection, whereas there were more cells in second division metaphase in the well-nourished (54.7%) than in the undernourished group (13.7%). Our results seem to indicate that malnutrition produces a prolongation in the time of proliferation of bone marrow cells during the lactation period. This agrees with other reports that used other cell systems in experimental animals and severely malnourished children.