In normal subjects Na, K, Cl and bicarbonate excretions were least during sleep, and increased progressively after waking to reach maxima at about midday. There was no corresponding change in the plasma level of these electrolytes. Inulin clearances showed that during sleep there was a small fall in glomerular filtration rate and a rise on subsequent waking, the mean night: day ratio for inulin clearance being 0.91; the corresponding ratio for creatinine clearance was 0.97. During sleep there was oliguria and the urine formed was of high sp. gr. and low pH. Ammonia excretion increased significantly during sleep and fell in the morning as bicarbonate excretion increased. Water diuresis, pituitary antidiuretic hormone and deoxycorticosterone acetate failed to alter the diurnal rhythm of electrolyte excretion, but administration of adrenocorticotropic hormone inverted the cycle of K excretion without affecting that of Na.--C. Gardikas.