A comparison of results of measuring supraoptic neuron activity was made in normal and in dehydrated rats by the following methods; (1) Measuring the nucleolar diameter of the neurons with a filar micrometer, with an image splitting eyepiece and, with the latter instrument, using the “comparative technique”—all three methods yielded statistically significant differences. (2) Measuring the concentration of cresyl violet in the nucleoli with a microspectrophotometer—a statistically significant difference was found. Also included for comparison were the results of a previous experiment in which the ribonucleic acid and succinic dehydrogenase content of the supraoptic nucleus was measured with a spectrophotometer following chemical extraction. The RNA was found to increase significantly but not the succinic dehydrogenase. Of the methods used for measuring nucleolar diameters the “comparative technique” appeared to be the most rapid from the practical viewpoint.