Sodium salts of medium chain fatty acids (MCFAs) enhance the absorption of hydrophilic drugs across the intestinal mucosa, but the mechanism behind the effect is largely unknown. In this study, the dose-dependent effects of the sodium salts of four MCFAs, C6 (caproate), C8 (caprylate), C10 (caprate) and C12 (laurate), on the permeability of the hydrophilic marker molecule [14C]mannitol were studied in monolayers of the human intestinal epithelial cell line, Caco-2, grown on permeable supports. C8, C10 and C12, but not C6, enhanced the permeability of [14C]mannitol in a dose-dependent manner. Comparison of the cellular effects of the MCFAs at concentrations that gave comparable (8.1- to 8.5-fold) absorption enhancement showed that: 1) C8 was active as absorption enhancer only when the tonicity of the medium was increased; 2) absorption enhancement mediated by C10 was related to a redistribution of the cytoskeleton and structural dilatations in the tight junctions; and 3) C12 was without effect on the cytoskeleton and cellular morphology. Studies on C10 under anisotonic conditions showed that deviations from isotonicity enhanced its effect. These results suggest that structurally similar MCFAs display dramatic differences in their mechanism of action. In addition, the effects of osmolality provide an explanation for the previously reported variability in the efficacy of MCFAs as absorption enhancers.