The effects of sodium caprate (C10) and disodium ethylenediaminetetraacetate (EDTA) on the epithelial permeability of polyethylene glycol 4000 (PEG4000) were examined in Caco-2 cell monolayers on the polycarbonate membrane (filter) of a Transwell cluster dish. Under serosal Ca(2+)-free conditions irrespective of the presence or absence of mucosal Ca2+, the permeability of PEG4000 was remarkably enhanced, even in the absence of enhancer. For control experimental conditions with a permeation enhancer in Caco-2 cells, Ca2+ was added to both the mucosal and serosal sides. Under the control conditions, EDTA increased the permeability of PEG4000 to 14 times the control and C10 by 3.5 times. Mucosal Ca(2+)-free conditions further increased the EDTA-enhancing effect, to 29 times the control value. In contrast to the EDTA effect, however, the C10 effect was unchanged in the absence of mucosal Ca2+. These results suggest that the EDTA effect is induced by chelation with Ca2+ but that C10 has some other action in addition to chelation.