Biological effects of increasing doses of human and salmon calcitonin (CT) have been studied in 17 patients suffering from Paget's disease of bone. Results show a dose response relationship on indirect indices of bone turn-over. The plateau phenomenon occurs with both CT and more rapidly with the lower dosage. There is also a transient increase in parathyroid activity during the period of time when bone turn-over is decreasing. Calciuretic effect was more pronounced with salmon than with human CT.