The mechanism for control of diurnal rhythm of water and electrolyte excretion remains obscure, though the fact that such a cycle occurs was reported by Quincke (1) in 1877 who pointed out that urinary output was appreciably higher during the day in contrast to night. Campbell and Webster (2,3) observed that the excretion of chlorides and urea was also higher during the day, though creatinine excretion remained more or less at the same level. Subsequent studies (Table I) by Kleitman (4), Simpson (5, 6), Norn (7), Manchester (8), Gerritzen (9), Brod (10), and Sirota, Baldwin, and Villarreal (11) confirmed the observations that urine flow and excretion of certain electrolytes decreased during the hours of deepest sleep in normal individuals.This rhythm was not reversed when normal subjects slept in the daytime and took food and worked at night as reported by Campbell and Webster (2, 3), and confirmed in studies of a night watchman by Jores (12). Gerritzen (13) studied the effect of exposure of four subjects to artificial daylight during the night, and darkness during the day. In three subjects duplication of the rhythm in water and chloride excretion resulted, with two maxima occurring at noon and at midnight. In the fourth subject the rhythm was completely reversed. Borst and de Vries (14) have pointed out the difficulty in evaluating the effect of daylight and of waking on diurnal rhythm, though in some of their subjects the response to the stimulus of daylight was prompt. Jores (12) reported the relative difficulty in reversing a rhythm by a reversal of routine, and this was further supported by Joslings' (15) observation that a subject on a boat going from east to west showed a peak urinary excretion one hour earlier every day, the peak remaining at the same hour on local time. Addis and associates (16) reported nocturnal depression of endogenous creatinine clearances.On the basis of U/P inulin ratios and creatinine clearance data, Sirota, Baldwin, and Villarreal (11) attributed the decrease in urine flow during sleep almost wholly to water reabsorption. The mechanisms of increased tubular reabsorption of water at night were not elucidated and significant diurnal variations in effective renal plasma flow were not noted (11).Reports of a reversal of renal function (Table II) began with Wilson's (17) observation that the day and night rates of excretion of water and solids were approximately the same in the presence of debility, and especially so in cardiac or renal disease. Quincke (18) confirmed these observations several years later, noting that patients with heart or kidney disease often had a peak of ditiresis during the night. Fishberg (19) attributed the nocturnal increase in urine flow in patients with nephritis and congestive heart failure to decreased renal venous pressure and increased cardiac output in the prone position. Subsequent work by Brod and Fejfar (20) and by Baldwin, Sirota, and Villarreal (21), employing clearance techniques, revealed an increased glomerular filtration rate and decrea...