the administration of crystalline insulin to thyroparathyroidectomized rats raised their calcium and phosphorus plasma levels. The plasma calcium gave a linear response between 31 and 250 mU of insulin/100 g body weight. The slope obtained is not significantly different from that obtained with parathyroid extract. The simultaneous administration of insulin and parathyroid extract to thyroparathyroidectomized rats affected the calcium and phosphorus plasma levels in an additive fashion. When chick embryo frontal bones were cultivatedin vitro with crystalline insulin the rate of resorption increased. Insulin increased the rate of glucose consumption by the explants and induced the accumulation of citrate in the culture medium. It is concluded that insulin stimulates bone resorption.