The urinary excretion of magnesium, calcium and sodium has been measured in 19 obese patients undergoing 12 days on complete fast (days 1-12) followed by 6 days of carbohydrate supplementation (days 13-18), while receiving no mineral supplements or either calcium, magnesium or sodium supplements. Magnesium and calcium followed different excretion patterns during the 12 days fast. The oral administration of 107 g carbohydrate daily during days 13-19 resulted in a marked decrease in the urinary excretion of magnesium, calcium and sodium by the non-, calcium- and sodium-supplemented patients, but there was no carbohydrate-induced reduction in magnesium excretion in the magnesium-supplemented patients. The kidney appears capable of selectively influencing calcium and sodium reabsorption, without magnesium excretion being affected. It is suggested that involvement in increased renal gluconeongenesis during fasting renders the renal tubule incapable of maximally conserving both divalent and monovalent ions. The increased tubular reabsorption of urinary constituents induced by carbohydrate administration probably stems from the concomitant reduction in gluconeogenesis.