The object of this study was to determine in 12 healthy subjects the relation between gastric secretion of acid and absorption of calcium from two different preparations of calcium, as judged from increased outputs of calcium in the urine. The increase in urinary output of calcium after solid calcium carbonate was greater in the subjects with the most gastric secretion of acid. The absorption of calcium after a solution of monocalcium citrate was independent of gastric secretion of acid. In the four subjects with the least gastric secretion of acid, there was no absorption of calcium after calcium carbonate, but the absorption after monocalcium citrate was as great as that for those who secreted greater amounts of acid.