Atrial natriuretic factor (ANF, Arg 101-Tyr 126; 0.5 and 2.5 micrograms) administered into the lateral cerebral ventricle of conscious euhydrated and dehydrated rats resulted in a significant reduction in the plasma vasopressin concentration. The effect of ANF on vasopressin secretion was greater in the water-deprived animal. Central ANF was without effect on mean arterial blood pressure in both euhydrated and dehydrated rats. The data suggest that ANF occurring in the central nervous system may be important in the control of vasopressin secretion.