1. Twelve patients with symptomatic Paget's disease were studied before starting treatment with salmon calcitonin (12.5 μg) subcutaneously twice daily. Eleven of them were studied again after 3 months on this therapy. 2. Although pretreatment values for urinary total hydroxyproline excretion and cardiac output were considerably increased in some patients, there was no correlation between these two variables in the group as a whole. 3. Treatment resulted in a striking reduction in disease activity; the mean urinary hydroxyproline decreased 67%. 4. There was, however, no significant fall in cardiac output or change in oxygen transport during treatment. 5. Of the eight patients with bone pain who received treatment, five claimed complete pain relief.