To determine the direct effects of cortisol on bone, rats were thyroparathyroidectomized (T(X)PT(X)), fed a low-calcium diet, and given high (50 mg/kg) or low (8 mg/kg) pharmacologic levels of cortisol with or without excess vitamin D3 (15,000 IU). Rats given vitamin D had osteoblasts and osteocytes interpreted ultrastructurally to be actively engaged in matrix synthesis, mineralization of matrix, and in calcium mobilization. Osteoclasts were numerous on metaphyseal trabeculae and in vascular channels of cortical bone. In T(X)PT(X) rats not given vitamin D, osteoblasts and osteocytes were interpreted to have reduced metabolic activity with minimal evidence of participation in bone formation or resorption. Cortisol at both dose levels failed to alter the electron microscopic appearance of osteoblasts, osteocytes, and osteoclasts with or without vitamin D. Bone turnover indicated by urinary hydroxyproline excretion was unaffected by cortisol treatment. These findings suggest that glucocorticoids have little direct action on bone cells and that their effects on calcium metabolism are probably mediated by an interference in intestinal calcium transport and by secondary hyperparathyroidism.