The superfusion of the hepatic portal vein of anaesthetized rats with hypertonic solutions of NaCl elicits a significant increase of neural activity measured in the hypothalamo-neurohypophysial tract. Superfusions of the liver or hepatic veins with hypertonic saline solutions had no effect, whereas superfusions of the portal vein also increased the firing rates of supraoptic endocrine neurones and the intramammary pressure. Results suggest that sodium chloride sensitive receptors (perhaps osmoreceptors) are located within the wall of the hepatic portal vein and mediate neurophypophysial hormone release.