Capric acid (C10) enhanced the absorption of cefoxitin sodium in a concentration-dependent manner following the rectal administration as a suppository in rats. The optimal concentration of C10 was 13%. C10 administered as a suppository also reduced rectal membrane resistance (R_m), showing that the above enhancing effect was induced by widening the paracellular pathway. Both the enhancing effect on the absorption and the reducing effect on R_m were inhibited by W7, an inhibitor of myosin light chain kinase. These results supported that, as shown in the in vitro Caco-2 cell system, the C10 effect on the paracellular pathway is due to activating the contraction of Ca²⁺-calmodulin-dependent actin filament.