Diurnal variations in excretion of sodium, chloride, potassium and water have been observed in several studies. Water and electrolyte excretion in normal individuals generally reaches a maximum sometime around midday with a minimum toward the end of the sleep period. These studies suggest that the daytime excretion rate exceeds the nighttime rate by 50 to 100 percent. Overnight urine collections have been used in many studies since they are easier to obtain, but they do not provide a direct estimate of the actual intake of sodium or potassium. In order to estimate actual intake, overnight values must be corrected to 24-hour values, that is, by multiplying the overnight values by previously determined ratios of 24-hour to overnight excretions. Preliminary studies by this group of investigators in hypertensives were the first which computed 24-hour to overnight ratios for excretion of sodium, potassium and creatinine in a definite way. This study in hypertensives showed a reversal of the diurnal cycle of sodium excretion. What is not clear in this study and others is whether the reversal is associated with hypertension or reflects decreased renal function with age or whether deterioration of renal function is due to the kidney readjusting its output of salt and water to maintain homeostasis in the face of both an excess sodium intake and a highly variable day to day intake. In addition, it may be that an abnormal diurnal pattern of sodium excretion may indicate that one is at risk of development of hypertension.