INVESTIGATIONS begun by one of us1 have been continued by further examination on the basis of Gomori's method2 and its modifications3 of the acid phosphatases of the pituitary. As substrate, α- and β-glycerophosphate was used, and for reasons emphasized by Michaelis4, the mixtures were stabilized with the acetate-veronal buffer and adjusted with a glass electrode. The sections (thickness 4 μ), first treated with citrate buffer at pH 5·5, were subjected to reaction mixtures with the following pH values: 6·8, 6·6, 6v3, 6·0, 5·5, 5·0, 4·5, 4·2, 4·1, 4·0, 3·9 and 3·5. In critical ranges the intervals were made shorter. The incubation times were 30 min., 1 hr., 2 hr., 24 hr., 48 hr. and 72 hr. Eighteen pituitaries of female and male guinea pigs were investigated. The process of diffusion as a possible source of errors was observed to be smaller than in the case of alkaline phosphatase. Some inconstancy of reaction already noticed and discussed by several previous authors6 did not interfere with the principal differences of localization and relative amounts of enzymes at various pH.