STudies will be pursued on several aspects of the endocrine control of water and sodium balances. Structural analogs of the neurohypophysial peptides will be prepared and their detailed pharmacological properties will be evaluated. The relations of specific molecular features to varied biological properties can be determined and analogs with desired new properties may be designed. Such analogs will also be used to help define the enzymic systems that metabolize the natural antidiuretic hormones. Studies on the evolution of mechanisms controlling the release of antidiuretic hormones will also be extended by using radioimmunoassay methods to follow plasma hormone levels among fishes and amphibians subjected to manipulations disturbing their osmotic and hemodynamic states.