Intravenous administration of porcine calcitonin or salmon calcitonin to a normal subject or to patients with Paget's disease of bone produces a marked decrease in the urinary excretion of hydroxyproline-containing peptides. The excretion of both polypeptides (retentate after dialysis) and oligopeptides (diffusate after dialysis) decreases after calcitonin. Since it has been shown previously in patients with Paget's disease that the retentate hydroxyproline reflects predominantly (bone) collagen synthesis, it is likely that bone collagen synthesis decreases acutely, accompanying the decrease in degradation of bone produced by calcitonin.