Fujino Spring is the foremost acid vitriol spring of Japan in ferrous and ferric ion contents. Six years ago the author reported that its internal use had an remarkable hemopoietic effect on hypochromic anemia and delayed the curve of alimentary hyperglycemia in rabbit. Twenty ml. of this vitriol water was diluted with plain water to two hundred ml. and was given to twenty one patients by stomach tube at 37°C. Gastric acidity of the patients was measured at 30 minutes intervals during the following 150 minutes after the administration of the water. In six cases of normal acidity no significant difference was proved between the effects of the mineral water and of caffein control (0.2g. in 200 ml.). In six cases of hyperacidity it seemed that the vitriol water decreased in the acidity curve compared with control. In nine patients of hypo- and anacidity both free and total acidity curve was elevated by the use of the mineral water compared with caffein control. The internal use of Fujino Spring in moderate dilution with plain water regulated the gastric acidity.