The location of adenosine triphosphatase in the brain has been studied in rapidly frozen-dried cerebral tissues of the Wistar rat. It is found that adenosine triphosphatase is an almost exclusively nuclear enzyme. Two tissue fractions of the cerebrum were separated, so that one sample was made up of vascular elements, and the other of neural elements. The two fractions were then studied for their adenosine triphosphatase activity, and compared with the histochemical findings. The two tissue fractions were found not to differ in the absence of bivalent cations. When Ca++ were added to the cerebral vascular suspension, ATPase activity was increased approximately 15 times, and only 3 times in the presence of Mg++. Conversely, the addition of Mg++ increased the ATPase activity of the neural fraction 200%; whereas, Ca++ was responsible for a 60% increase. This fact was detectable microscopically when Ca++ was found to intensify vascular nuclear staining, and Mg++ to increase the neuronal and glial nuclear staining. The results, histochemical and biochemical, are mutually confirmatory.