Inulin space and thiocyanate space determinations were made on three groups of male rats: the first group was chronically undernourished by feeding a restricted ration for 10 weeks, the second group was starved with free access to water, and the third was made up of normal male controls. The inulin space was found to expand relative to the final body weight or lean body mass in the undernourished animal. No absolute increase in inulin space took place. In starvation the inulin space did not change in proportion to the body weight and its absolute value decreased. The thiocyanate space relative to the final body weight or lean body mass expanded slightly but significantly in chronic undernutrition and in starvation, but no absolute increase of the thiocyanate space took place.