Medium chain glycerides (MCGs) have been reported to enhance intestinal absorption of hydrophilic drugs. However, the mechanisms involved in absorption enhancement are not well understood. The effects of MCGs (CapMul MCM) on physiological properties of rabbit ileum and distal colon, including active ion transport, transepithelial resistance (Rt) and passive permeability, have been investigated in vitro. CapMul MCM inhibited active ion transport (measured as a decrease in short-circuit current, Isc) in both intestinal segments in a concentration-dependent manner. The inhibition of Isc was rapidly reversible (within 100 min) upon removal of CapMul MCM. The data indicate that CapMul MCM preferentially affected ion transport by villus cells in the ileum and surface cells in the distal colon. Ion transport in crypt cells in both segments was not significantly altered. Rt of the ileum was not significantly affected by 5% CapMul MCM, while mannitol transport was 6 fold enhanced. Treatment of distal colon with 1% CapMul MCM reduced Rt by 95%, while mannitol transport was 100 fold enhanced. In a parallel experiment, mucosal(m)-to-serosal(s) transport of cephalexin, a beta-lactam antibiotic, in the ileum was about 40% reduced in the presence of 5% CapMul MCM, whereas transport in the s-to-m direction was 2.5 fold enhanced. Treatment of the distal colon with 1% CapMul MCM resulted in 25 fold enhancement of cephalexin transport in either direction. These results suggest that absorption enhancement by MCGs results from an increased permeability of the intestine confined to the villus or surface epithelium.