Activity in hypothalamic supraoptic nuclear cells was recorded with microelectrodes in hemispherectomized, chloralose-anesthetized cats. Repetitive stimulations of skin afferents augmented activity of some units but inhibited others. Activity of units was little affected by decortication and removal of the caudate nucleus but considerably increased by decerebellation, midbrain transection and separation from thalamic connections. Stimulation of the stalk augmented rate of firing of nuclear units. Complete isolation of the supraoptic nuclear region from all neural connections except with hypophysis resulted in an increase in cellular discharge rate which lasted for many hours. This augmented activity was associated with a decreased urine output until the pituitary stalk was sectioned. The supraoptic nuclear cells in the island retained their sensitivity to osmotic changes in the blood produced by intracarotid injection of hypertonic glucose.Stimulation of the cerebellar cortex by application of glutamic acid increased urine output. These experiments provide evidence that the supraoptic nucleus cells in addition to being subject to excitation by both neural and humoral agents are under inhibitory control from structures within the cerebellum, midbrain and thalamus.