In situ perfusions into the small intestine of rats showed the absorption kinetics of nitrate and nitrite. This absorption is important since the rate rises to 50% for nitrates and 80% for nitrites in our experimental conditions. These compounds are excreted in the biliary secretion and, with the nitrate perfusions, nitrite appearance in the perfusion medium indicates some reduction potentiality either in the liver or in the entero-cells. These results are all the more interesting as men daily ingest nitrates and nitrites with many foods.