In 94 of 111 dogs, stimulation of the hypothalamus resulted in moderate to marked antidiuretic effects. The onset was slow (1/2-3 min) and the effect lasted for 15-35 min. These antidiuretic responses could be elicited by stimulating not only the points corresponding to supraoptic and paraventricular nuclei, but also a larger area in the hypothalamus. The antidiuretic action evoked by 20-sec excitation with stimuli of 7V and 50 cycles/sec was roughly equivalent to that following intravenous administration of 2.5 mU/kg of Pitressin. Atro-pine and d-hyoscyamine did not block the antidiuretic responses to hypothalamic stimulation, but adequate amounts of n-ethyl-nortropin-benzhydrylether-hydrobromide and Dibenzyline did. The possible mechanism of such a phenomenon is discussed.