A nucleus of the hypothalamus lying in the supraoptic portion. Its axons, along with those of the supraoptic nucleus, form the supraopticohypophyseal tract. The cells of this tract regulate water balance in the body, secreting vasopressin, the antidiuretic hormone, which is stored in their axon terminals in the posterior lobe of the pituitary gland. Its cells also seem to stimulate the sensation of thirst. Finally, the supraoptic and paraventricular nuclei produce oxytocin, which causes uterine muscle contraction and milk secretion.