In the isolated tibia of the cat, perfused with blood from its own carotid connected to popliteal artery via a pump, infusion of porcine thyrocalcitonin causes an arteriovenous difference in calcium concentration of up to 5%, corresponding to net retention of calcium in bone.2. When taken in conjunction with determinations of radiocalcium clearance in the tibia, the effect is of the right order of magnitude to account for the hypocalcaemic action of thyrocalcitonin in the whole animal.3. Calcium retention was accompanied by a small increase in acidity of outflowing venous blood whereas verification of the hypothesis of calcium regulation by pH change in the bone substance would require a decrease.
