The present report is primarily concerned with the polyuria or experimental diabetes insipidus, observed during an investigation of the effects of lesions in the hypothalamus of the cat, and with the attempt to localize the precise nuclear groups within this region which are involved in the regulation of water exchange. The mammalian hypothalamus has been held to be the chief regulating center for a large number of vegetative processes, such as water, fat and carbohydrate metabolism, the regulation of temperature, pupillary dilatation, the regulation of blood pressure and so on, but, aside from the work of Ranson and Magoun1 and of Kabat, Magoun and Ranson,2 who obtained marked respiratory acceleration, rises in blood pressure and pupillary dilatation by stimulation of the lateral hypothalamic area and the region around the fornix, and the work of Broers,3 who showed that destruction of the supra-optic nuclei results in diabetes insipidus,