Analysis of the principal routes of calcium metabolism by means of calcium-45 in 26 patients suffering from Paget's diseases sometimes showed, in agreement with data from the literature, a considerable increase in the common stocks of calcium in 25 cases, and an elevation in the anabolism (Vo+) in bone and in the catabolism (Vo-) in bone in all cases. The calcium balance was sometimes equal, but often clearly positive (10 cases) or negative (10 cases) and the level intestinal absorption of calcium was usually within the normal limits. An elevation in the endogenous fecal calcium (Vf) noted in half of the cases, although no value was below normal, seemed to represent an metabolic anomaly of abnormally high frequency in patients with Paget's disease, even though this fact has never been emphasized before. Statistically significant correlations were shown between the values of different kinetic and metabolic parameters in the group of patients studied. A second kinetic study carried out after several weeks of treatment with porcine calcitonin showed a statistically significant diminution in the values of Vo +, Vo -, endogenous fecal calcium, calciuria, hydroxyprolinuria, and in alkaline phosphatasaemia, as well as significant variations in the calcium balance in a positive direction. The common calcium stocks did not show a significant diminution, contrary to the findings of other authors. The level of intestinal absorption of calcium increased more often than it decreased, but the magnitude of the variations was slightly less than the level of statistical significance.