In order to investigate the effect of calcitonin (CT) on calcium and phosphorus metabolism in primary hyperparathyroidism (PHP), porcine calcitonin (80 MRC units) was injected intramuscularly at 9:00 a.m. and 5:00 p.m. for 10-14 days in 7 patients with parathyroid adenoma. Fasting blood specimens were drawn at 8:00 a.m. every other day and 24 hour urine samples were collected through out control and test days. To examine the acute effect of CT, blood and urine were checked several times until 8 hours after the first injection. A fall in the fasting serum calcium level observed in 5 patients during the repeated administrations of CT, as well as that observed in 6 patients within 6 hours after the first injection, showed a significant correlation with the initial serum calcium level. Serum phosphorus concentration decreased in all patients 6 hours after the first injection, while fasting levels seemed to remain unchanged. During the repeated administrations, urinary excretion of calcium and phosphrus decreased correspondingly with the fall in serum calcium levels, although no definite tendancy was observed within 8 hours after the first injection. Fasting serum PTH levels during the repeated administrations were measured in 2 patients. In a patient whose serum calcium returned to the initial level on the 7th day of administration, a gradual rise of PTH was observed, while in another patient whose serum calcium was kept lower than the initial level, PTH remained almost unchanged. These results indicate that, under such a condition where there is marked increase of bone resorption as PHP, repeated administrations of CT bring about not only a hypocalcemic effect but also the reduction of calcium and phosphorus excretion through a decreased filtered load. In addition, it was suggested that, in some cases of PHP, the hypocalcemic effect of CT may be abolished by an increase of PTH secretion from the parathyroid glands during long-term administration.