1. Absorption of carbohydrate probe molecules from ligated loops of rat small intestine was studied. Absorption was determined by measuring recovery of molecules in the urine, corrected for incomplete recovery after intravenous injection, and was examined for correlation with several parameters of molecular dimension.
2. Absorption depended on molecular volume rather than relative molecular mass, molecular radius or molecular area.
3. Molecules with a molecular volume below 225 × 10−3 nm3 were absorbed to a greater extent than larger molecules, and absorption was affected critically by molecular volume, small changes in volume producing considerable variation in absorption.
4. Absorption of larger molecules was not affected by changes in volume within the range 362 × 10−3–1128 × 10−3 nm3.
5. These findings support the concept that there are at least two aqueous diffusion pathways across the intestinal mucosa, small molecules diffusing through a small channel of finite dimension, compatible with a transcellular aqueous pore, whilst large molecules diffuse through a less frequent pathway of considerably larger dimensions.