Release of antidiuretic substance (vasopressin) from the central end of the cut pituitary stalk and the urine flow rate were estimated during electrical stimulation of the CA₁ field of the left hippocampus in rats. Frequencies of 4, 12, or 36 c/sec were used for hippocampal stimulation during the collection of the antidiuretic substance released from the cut infundibular axons, and frequences of 4 or 36 c/sec were used for hippocampal stimulation during urine flow determination. In all experiments, the current and voltage were adjusted to obtain approximately the same energy per train of pulses (2,160 nanowatt seconds). The antidiuretic activity of the fluid washing the cut pituitary stalk decreased during 4 c/sec hippocampal stimulation, showed no significant change during 12 c/sec stimulation, and increased considerably during 36 c/sec stimulation. The rate of urine flow did not alter during hippocampal stimulation at 4 c/sec, but decreased markedly during 36 c/sec stimulation. It is assumed that the hippocampus is able to trigger an increase or decrease in the rate of axoplasmic flow of antidiuretic substance and in the release of antidiuretic hormone from the pituitary gland. The frequency of hippocampal stimulation determines the response when the energy per train of electrical pulses remains constant.