Reducing the cerebrospinal fluid concentration of NaCl by infusion into a lateral cerebral ventricle of isotonic solutions of 0.3 mol/l mannitol or sucrose at 1 ml/h for 2 h caused a large reduction in renal Na excretion in conscious sheep deprived of water for 24 or 48 h. Infusion of 0.3 mol/l mannitol into the lateral ventricle of water-replete sheep did not alter renal Na excretion but induced a water diuresis. These data indicate that during dehydration, renal Na excretion may be under the control of the brain. The pathway(s) from brain to kidney mediating this antinatriuretic effect are not known.