In the present paper an attempt has been made to map out, in the central nervous system of the dog, the distribution of the enzyme (or enzyme system) which synthesizes acetylcholine. It has been shown that this enzyme can be extracted from acetone-dried tissue (Feldberg & Mann, 1944) and that such extracts, when incubated aerobically, form large amounts of acetylcholine in the presence of KCI, MgCl2, choline, cysteine, citrate and adenosine-triphosphate (Nachmansohn & Machado, 1943; Nachmansohn, John & Waelsch, 1943; Nachmansohn & John, 1945; Feldberg & Mann, 1944, 1945, 1946; Feldberg & Hebb, 1947). Since this method is applicable to very small amounts of tissue, it was possible to compare the enzyme concentration in various quite small regions of the central nervous system.