We studied the effect of an Mg-deficient diet (20 mg Mg/100 g diet) on the nutritive utilization of dietary protein in Wistar rats after 7, 35, 42, 49, 63, and 70 days. Magnesium deficiency significantly increased fecal and urinary excretion of nitrogen, leading to a decrease in apparent absorption and balance of nitrogen. In Mg-deficient animals, the plasma concentration of total protein was significantly lower than in control rats. The results indicate that Mg deficiency can reduce the nutritive utilization of protein, an effect that may contribute to the appearance of the malnutrition syndrome in rats given an Mg-deficient diet.