We have confirmed the serum calcium-raising effect of adrenalectomy in young male rats 5-6 h after parathyroidectomy that was first observed by others many years ago. (The phenomenon has also been reported in cats, dogs, and mice.) In addition, we have shown that adrenalectomy raises the serum ionized calcium as well as total calcium and that the effect occurs in young female as well as in young male rats. Furthermore, we have found that the serum calcium-raising effect of adrenalectomy occurs if the adrenalectomy is performed several days before parathyroidectomy or 6 h after parathyroidectomy, as well as at the same time as the parathyroidectomy. When the rats were adrenalectomized 7-9 days before parathyroidectomy and given small daily life-maintaining doses of corticosterone or cortisone acetate, this glucocorticoid treatment did not reverse the adrenalectomy effect. This led us to think at first that the effect of adrenalectomy must be due to the loss of an unknown adrenal factor rather than to loss of glucocorticoid. Additional experiments, however, in which corticosterone or hydrocortisone was administered by continuous release pellets, demonstrated conclusively that a small continuous supply of corticosterone (within the physiological range as determined by immunoassay of plasma) was sufficient to reverse the adrenalectomy effect. The results with hydrocortisone were similar at even lower doses than of corticosterone. Somewhat higher doses of corticosterone or hydrocortisone reduced the serum calcium even below the parathyroidectomy level. In a preliminary investigation of the specificity of the glucocorticoid effect we found that aldosterone, dehydroepiandrosterone, or estradiol had no effect on serum calcium under similar conditions. We conclude that the fall in serum calcium after parathyroidectomy in rats is due in part to the hypocalcemic effect of endogenous corticosterone. Thus, the loss of corticosterone after adrenalectomy explains the serum calcium-raising effect of adrenalectomy in parathyroidectomized rats. These results also suggest that glucocorticoids at physiological levels have a significant effect on calcium metabolism in general.