1. The recent recognition of the metabolic, as opposed to absorptive, functions of the small intestine prompted efforts the improve the preparation of metabolically competent columnar absorptive cells ('enterocytes') and to study their metabolic properties. 2. With this preparation, linear rates of O2 consumption are obtained for 40 min at 37 degrees C that are more than 50% higher than rates reported by other authors. 3. Among added substrates, glucose, glutamine and glutamate are the preferred fuels of respiration. The main nitrogenous products of glutamine metabolism are NH3, alanine and glutamate. Glutamine carbon was not detectable in citrulline or proline, in contrast with the findings of Windmueller & Spaeth [(1974) J. Biol. Chem. 249, 5070-5079] in the vascularly perfused small intestine. 4. The rates of O2 uptake in the presence of glutamine or glutamate are sufficient to account for the formation of the carbon skeleton of alanine from the amino acid substrate, i.e. the ratio of O2 used/alanine formed is greater than 1.5. 5. Added ADP and ATP are rapidly degraded to AMP and IMP to a large extent by release of hydrolytic enzymes from the enterocytes into the medium. 6. Chicken enterocytes isolated by the same method are more stable; linear rates of O2 uptake are maintained for 60-70 min.