1. The kinetics of absorption of glucose, galactose and alpha-methyl glucoside have been measured in rat jejunum in vivo using a chemical method and a new electrical technique. 2. Sugar absorption estimated by chemical methods exhibited two components. One component was phlorrhizin-sensitive, saturable and generated electrical potential differences (electrogenic active component) while the other was phlorrhizin-insensitive, non-saturable and did not generate electrical potentials (diffusive component). 3. The diffusive component of the actively transported sugars was indentical to the absorption behaviour of sorbose, a hexose that is not actively transferred. 4. A method for correcting the data obtained from chemical absorption studies for the diffusive component was developed. The corrected, operational kinetic constants for 'apparent Km' obtained by this method were not significantly different to values obtained electrically. The identity between the values obtained by both methods supports the concept that they represent a measure of the same rate-limiting step in the absorption process. 5. The application and significance of the techniques is discussed in relation to the clinical assessment of intestinal sugar absorption.