1. The absorption in vivo of d-galactose by the rat small intestine has been examined in proximal jejunum and distal ileum by use of a recirculation—perfusion technique.
2. Multiple sequential perfusions over 4 h produced no subsequent functional or morphological damage in the perfused segments.
3. Absorption of galactose from 8 and 64 mmol/l solutions was found to be independent of flow rate over the range 1·0–6·5 ml/min.
4. Galactose absorption in both the jejunum and the ileum exhibited saturation kinetics of the Michaelis—Menten type, and phlorrhizin sensitivity. Sorbose was only absorbed minimally. These observations demonstrate that galactose is absorbed by carrier-mediated transport and that there is no significant passive diffusive component in vivo.
5. Under the stated experimental conditions, the maximum absorptive capacity was 4·5 times greater in the jejunum than in the ileum. The Michaelis constant for galactose was higher in the jejunum than in the ileum.
6. Enterocytes were isolated from perfused segments and quantified by DNA assay with a correction for yield. In this manner, the absorptive capacity per enterocyte was calculated.
7. The maximum absorptive capacity per enterocyte was 3·5 times greater in the jejunum than in the ileum.