Vitamin D and cyclic AMP changes in response to calcitonin injection were studies in three normal adults and one patient with osteogenesis imperfecta. Plasma 1,25-(OH)2D levels increased in one normal adult and the patient. Plasma 25-OHD levels did not change. Plasma cyclic AMP was decreased in all four subjects, but urine cyclic AMP did not change. These results suggest that calcitonin may affect plasma cyclic AMP and may have some affect on renal la-hydroxylation in humans.