Comparative studies were made with animals with normal and animals with lowered blood-Ca. Former studies have shown the antagonism between Mg and Ca. The precipitation of Ca in the blood with Na-oxalate led to muscular twitchings. Magnesium lowers the threshold of stimulation in voluntary muscles and stops twitchings. The opposite effect is found in involuntary muscles. Dogs were injected with 20% MgSO4 in one application which rarely lasted over 1 [image] min. A few minutes before the injection a few cc. of blood was drawn from the veins, or in a few cases from the heart, and tested for Ca. The fatal dose for a dog with normal Ca was 0.23 to 0.28 gm. per kgm. body weight. Dogs with hypercalcemia induced by the injection of the parathyroid hormone tolerate 0.30 to 0.32 gm. of the MgSO4 and are killed at about 0.35 gm. Dogs with hypocalcemia induced by parathyroidectomy or by the injection of Na oxalate were killed with the injection of 0.14 to 0.19 gm. of MgSO4. The authors found that the parathyroid hormone had a marked effect on the heart and vascular system and that this effect was antagonized by the injection of MgSO4.