in some cases there is an absence of hydrochloric acid.Hardt1 in 1916 called attention to the fact that thyroid feeding caused lowered gastric acidity in normal dogs. Barker 2 in 1918 mentioned hypochlorhydria, and King 3 in 1919 noted that achlorhydria and achylia occur in hyperthyroidism. Lockwood 4 in 1925 and Moll and Scott5 in 1927 reported a high incidence of achlorhydria in exophthalmic goiter. Following this line of thought, Truesdell 6 in 1926 showed that a lowering of acidity could be produced regularly by feeding thyroid to dogs, and in some achlorhydria was produced.On the other hand, Crile 7 and his co-workers have held that the same mechanism that produced hyperthyroidism could in some cases produce peptic ulcer.He has also stated that denervation of the suprarenals and a hemithyroidectomy frequently relieves recurrent peptic ulcer. If this theory is true, hyperthyroidism should raise, not lower, the gastric acidity. Lerman and Means8 have shown, however, that there is a defi¬ nite tendency to lowered gastric acidity in hyperthy¬ roidism and that the proportion of achlorhydria is increased beyond the normal.This study was undertaken to show the relationship between hyperthyroidism and gastric acidity and to determine to what extent the acid returned to its nor¬ mal level after relief of the toxicity. Also, we wanted to see what effect a postoperative lowering of the basal metabolic rate below the normal level would have on gastric acid. This study comprises the results of gastric analyses in 100 cases of hyperthyroidism, taken before opera¬ tion. At the Lahey Clinic it is the practice to have all patients return three months after partial thyroidectomy for a check-up metabolism. Of the original 100 cases, we were able to get only about one half back for check-up gastric studies. This was due partly to the reluctance of some patients to have a second test, partly to inability to return, and partly to the fact that three months has not elapsed since some of the original tests were made. It was felt, however, that for purposes of averages, a comparable number of postoperative check¬ ups would give as much information. Consequently, determinations were made in 114 cases. In addition.