Twenty four young (mean age 29-2 years, range [25][26][27][28][29][30][31][32][33][34][35] and 21 elderly (mean age 66-5, range 60-80) healthy subjects collected their urine in timed aliquots over 24 hours. The elderly subjects had been selected for their fitness by clinical and laboratory examinations and all lived independently at home. Sodium and potassium excretions were reduced in the elderly subjects compared with the young subjects, potassium excretion considerably so. This was despite similar 24 hour urine volumes and total solute excretion by both groups.The ratios of rates of excretion of water, electrolytes, and solutes during the night to the rates of excretion during the day were found to be higher in the elderly than the young subjects.Reduced day to night ratios of urinary excretion may be partly responsible for complaints of nocturia and sleep disturbance in elderly people. Introduction For many years it has been known that urine flow is lower at night than during the day in healthy subjects.' There is also an accompanying nocturnal reduction in electrolyte excretion.2 It has been speculated that reduction in urine flow at night in mammals may have evolved to permit undisturbed sleep.3Several studies of excretory rhythms in old patients have been reported.4-9 The first study was of patients in psychiatric hospitals4 5; the second of elderly patients in hospitals long term6; the third of nine patients in hospital, only two of whom were

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A ging is a normal degenerative biologic process that affects many organs, of which the kidney is one of the most prominently affected. Physicians who treat elderly patients must be aware of the decline in renal function that accompanies the aging process. The findings of urinary abnormalities or a decreased glomerular filtration rate (GFR) in elderly patients should alert clinicians to fully evaluate the renal function. GFR decreases steadily, starting in the middle age. Evaluation of elderly patients should include careful consideration of possible fluid and electrolyte disturbances. Acute renal failure is most frequent among the aged, mainly due to the comorbidities and the intervention they require. Diabetes and renovascular diseases including hypertension now account for most of the cases ofchronic kidney disease. The elderly are the fastest growing population of patients requiring renal replacement therapy in this aging society. This represents one of the great public health issues.