3 p. 100 of Maillard's reaction products (MRP), obtained by mild heating of glucose and glycine, were added to a semi-synthetic diet sterilized by irradiation. This resulted in increased dry matter excretion, more marked in axenic than in holoxenic rats, and in reduced apparent absorption of sodium and potassium in holoxenic rats. The addition of these products caused diarrhoea in axenic animals, thus preventing a good estimation of apparent absorption. It led to a significant decrease in the retention of calcium, phosphorus, magnesium and copper only in the axenic rats. It is shown that MRP affect mineral metabolism and that the microflora plays a protective role in the physiology of digestion.