PROTOCOL OUTLINE: Participants undergo a series of tests to determine the presence, absence, cause, natural history, clinical status, and mode of inheritance of their type of diabetes insipidus (DI). The studies include measurements of basal fluid intake and urine output, plasma vasopressin during standard fluid deprivation or waterload/saline infusion tests, and changes in water balance during a therapeutic trial of DDAVP. If clinically indicated, echocardiograms and assays of plasma catecholes and renin are also completed.