The evolution of the nutritive utilization and plasma and muscular content of calcium and phosphorus in rats is studied, as well as the influence that hydrocortisone acetate has (4 mg/100 g weight and day), on them. In pregnant rats injected with saline solution, the retention of calcium is higher on the days 15-18 as a consequence of the increase on the net absorption of the cation. The balance of phosphorus reaches its maximum hight on days 18-21 of gestation, due to the decrease of the urinary excretion during these days. The administration of hydrocortisone acetate to pregnant rats originate a decrease of the net absorption and an increase of the urinary excretion of calcium and phosphorus, which leads to significantly lower balances than those of animals injected with saline solution. The hormone maintains the animals in a negative balance of phosphorus until day 18 of gestation. Besides, it impedes the high retention of calcium which is found with the control pregnant rats on days 15-18. In general, the content in calcium and phosphorus by gram of dry matter of Longissimus dorsi muscle is not modified during gestation. From day 9 of the treatment with hydrocortisone acetate a significant decrease of calcium and phosphorus of the muscle is observed, that in calcium is the more marked the more the treatment is prolonged. This effect seems to be due to the catabolic action of the hormone.