The distribution of adenosine triphosphatase was studied in morphologically characterized subcellular fractions of guinea-pig brain. The conditions of homogenization were selected so as to favour the survival of nerve endings as organized structures. 2. A fraction consisting mainly of the external membranes of nerve endings was rich in a ouabain-sensitive Na(+)-K(+)-stimulated adenosine triphosphatase which closely resembled that present in the classical microsomal fraction studied by other workers, but which showed a higher specific activity. 3. A dinitrophenol-stimulated adenosine triphosphatase was located in the nerve-ending mitochondria. 4. The synaptic-vesicle fraction contained a small amount of adenosine triphosphatase that differed in its response to several ions and other compounds from the membrane, myelin and mitochondrial fractions, indicating freedom from contamination by these elements.
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