—(1) ATP: creatine phosphotransferase of rat cerebral cortex is soluble to the extent of 57 per cent when the tissue is homogenized in 0.25 M-sucrose and 80 per cent when distilled water is used for tissue dispersion. Among particulate fractions, the crude mitochondria] fraction contains the highest percentage of enzyme activity.

(2) Discontinuous sucrose gradient fractionation of the crude mitochondrial fraction yields about 55 per cent of the particulate activity in the nerve ending fractions and 24 per cent in the mitochondrial pellet.

(3) Rupturing of the nerve-ending particles by a moderate osmotic shock designed to spare the mitochondria results in about 60 per cent of the ATP:creatine phosphotransferase becoming soluble, the remainder preserving the association with heavy particles, presumably mitochondria.

(4) Subfractionation of the microsomal fraction on a discontinuous sucrose gradient reveals that this particulate component of the enzyme is an adsorption artifact.

(5) The overall evidence points to at least two distinct subcellular localizations of the enzyme in rat brain cortex, a major soluble component and a particulate component. It has not been unequivocally shown whether the latter, in turn, reflects the presence of a single, mitochondrial component or whether the soluble matrix of the nerve ending particles represents a third locale for the enzyme.