Secretion of the neurohypophyseal principles arginine vasopressin, oxytocin, vasopressin-neurophysin, and oxytocin-neurophysin by hypothalamic-neurohypophyseal explants in organ culture was studied. The viability of explant cultures was tested on day 6 by their ability to incorporate 3H-amino acid into acid-precipitable protein, specifically vasopressin-neurophysin. The explants released all four neurosecretory products in a controlled manner during 8 successive days of culture. In addition, these substances were all present in neural lobes and hypothalami at the end of the culture period. Although the tissue retained a 1:1 molar ratio between the hormone and corresponding neurophysin, the levels of neurophysins in culture media were much higher than the levels of hormones. Through immunoprecipitation techniques we determined that both arginine vasopressin and oxytocin were unstable in culture media containing explants or in medium containing 10% heat-inactivated fetal calf serum. This suggests that neurophysins are more stable indicators of secretion by explants than are the corresponding hormones. An increased release of arginine vasopressin, oxytocin, vasopressin-neurophysin, and oxytocin-neurophysin occurred in response to elevated K+ and to increased sodium ions in the medium, and these four substances were released in different molecular proportions. The results indicate that explant cultures may be useful for comparisons of secretion of the two neurophysins and, to a more limited extent, of the two hormones in response to such stimuli.