Four obese patients were given a single-meal diet (684 kcal.) for two periods of three days each. Water-loss, according to Peter-Passmore formula, and urinary sodium and potassium excretion were measured at 4-hours intervals. A water-loss greater in the first than in the second and in the third day in both periods and a strong linear correlation between water-loss and sodium urinary excretion were found. Furthermore a circadian rhythm either is sodium or in potassium urinary excretion not modifiable by meal timing with both regiment was detected.