The effect of continuous infusion of calcitonin on serum osteocalcin concentration was studied in 14 patients with Paget's disease. In all patients serum osteocalcin was initially increased. Within 24 h calcitonin gradually reduced serum osteocalcin, a marker of osteoblastic activity. This means that inhibition of the function of the osteoclasts by calcitonin results in an inhibition of the osteoblasts within 24 h in patients with Paget's disease.