The inability to excrete an oral or parenteral load of water in a normal manner is a characteristic of the adrenalectomized or hypophysectomized animal and of the patient with adrenal or pituitary insufficiency ( 1-9). This defect persists in the presence of a normal salt balance, extracellular volume, and an adequate supply of salt-retaining hormone, desoxycorticosterone acetate (DCA) or aldosterone (4,7,8,10,11), but it can usually be corrected by the administration of hydrocortisone-like steroids (3,4,7,9).The basic mechanisms responsible for impaired water tolerance in these diseases have not been adequately clarified, nor have the processes by which hydrocortisone causes a normal water diuresis to occur.Gaunt, Birnie and Eversole (8) and others (7, 12) have suggested that a homeostatic "balance" exists between the antidiuretic hormone (ADH) of the neurohypophysis and the 11, 17 hydroxylated steroids of the adrenal cortex. They suggest that in the absence of these steroids, a relative or absolute excess of ADH exists, preventing a normal water diuresis. To prove this hypothesis, a sustained level of secretion of ADH should be demonstrated after the adrenal or anterior pituitary deficient animal has been water loaded, a procedure which ordinarily inhibits ADH secretion. To date, no such sustained activity has been observed using a generally acceptable and specific assay technique (6,13,14). Furthermore, the patient with adrenal or pituitary insufficiency normally inactivates and is not hypersensitive to exogenous antidiuretic hormone (Pitressin®) (6). Recent studies have stressed the importance of nonhormonal factors in determining the characteristics of water diuresis in normal subjects. The rate of glomerular filtration and solute excretion, and the distribution of the reabsorbed solutes between the proximal and distal tubules can profoundly alter the diuretic response to water in the absence of circulating ADH (15-21). Reduced glomerular filtration and renal blood flow are usually present in subjects with pituitary and adrenal insufficiency. In the latter, and occasionally in the former group, tubular reabsorption of sodium is impaired. Welt (22) postulated that the impaired water diuresis of these states is related primarily to alterations in the nonhormonal factors rather than to any disturbance in the secretion, metabolism, or action of ADH. If this theory is correct, it should be possible, while these subjects are under the influence of a sustained water load, to improve considerably their response to water loading by techniques which increase glomerular filtration rate and alter the distribution of solute (sodium) reabsorption between the proximal and distal tubule.This approach formed the basis of the present study. Aminophyllin was used to increase glomerular filtration acutely and alter the tubular reabsorption of solute. Its effect was compared in the same subject with the subsequent administration of hydrocortisone. The results indicate that the marked improvement in the execretion of water which...