It is well established that primary and secondary deficiency of cortisol secretion by the adrenals causes a grossly impaired diuresis during the first 4 h after an oral water load, and that the levels of blood cortisol affect the osmotic threshold for secretion of antidiuretic hormone (ADH) by the neurohypophysis (Aubrey, Nankin, Moses & Streeten, 1965). The changes in blood cortisol concentration after a water load, however, have been less thoroughly studied, though Hatfield & Shuster (1959) found a fall in plasma cortisol levels, as measured by the Porter—Silber reaction. Since a variation in cortisol concentration might itself affect diuresis after a water load, such a response would be important in the understanding of the physiology of diuresis and was, therefore, re-investigated. The experiments were performed on six healthy adult male subjects. No food or fluids were taken from 22.00 h the previous night. At 09.15 h a sample of