Studies carried out during the preceding project period have shown that a) the osmotic threshold for antidiuresis in man and toad are similar, b) isolated preopticoneurohypophyseal systems of toads release increased amounts of vasotocin upon osmotic stimulation in short-term organ cultures, and c) changes in interstitial fluid tonicity markedly alter the responsiveness of isolated toad bladders to vasotocin. During the next project period we hope to learn a) whether agents and conditions which alter water balance in man alter the osmotic threshold for antidiuresis in the toad, b) which steps intervene between osmoreceptor activation and vasotocin secretion in organ culture, and c) how the vasotocin target cell adjusts its responsiveness to hormone with changes in plasma osmolality. The osmotic threshold of intact toads will be determined by immersing animals in saline solutions of increasing tonicity, pithing them, and rapidly removing and fixing the urinary bladders with 1% glutaraldehyde. The hydraulic conductivity of these fixed bladders (as measured under standard conditions in a tissue bath) will be related to the osmolality of the plasma collected just after pithing the toad. Organ cultures of the toad's preoptico-neurohypophyseal system will be stimulated with hypertonic sodium chloride and isotonic 55 mM KCl media in the presence of potential antagonists and synergists of the secretory process; the amount of vasotocin released into the media in response to these stimuli will be estimated with the hydroosmotic toad bladder bioassay. Changes in the responsiveness of isolated toad urinary bladders to vasotocin with changes in interstitial fluid osmolality will be assessed by measuring the osmotic water flux, urea movement, and active sodium transport across the bladder epithelium.