1. The effects on renal electrolyte excretion of human and salmon synthetic calcitonins were studied in the rat.
2. As little as 50 ng of salmon calcitonin produced an increase in sodium excretion, but human calcitonin in doses up to 15 μg had only a slight effect.
3. Small doses of both hormones decreased urinary calcium and magnesium. Larger doses of salmon calcitonin produced less depression of calcium and magnesium excretion even in the presence of a greater fall in plasma calcium.
4. The mechanisms responsible for these effects and the possible therapeutic implications are discussed.