In their summary of changes in renal function with age, Porush and Faubert do not mention one of the most troublesome and least understood: the reversal of the circadian rhythm of water and electrolyte excretion. Young adults normally excrete the greater part of their 24-hour urine volume during the day and have a low night output. This rhythm has been shown to weaken by the age of 50 years and to be lost or apparently reversed by the eighth decade. Increased urine production is probably the principal cause of the need to get up at least once at night experienced by practically all elderly people. Together with bladder hyperreflexia, it is also an important cause of nocturnal incontinence.