In the present study, we clarified how the NaCl gradient across the epithelial cells regulates the paracellular ion conductance. Under isotonic conditions, the absorption-directed NaCl gradient elevated the paracellular conductances of Na(+) (G(Na)) and Cl(-) (G(Cl)), while the secretion-directed NaCl gradient diminished the G(Na) and G(Cl). We further investigated the paracellular ionic conductances of NMDG (G(NMDG)) and gluconate (G(gluconate)) by replacing Na(+) with NMDG or Cl(-) with gluconate. The G(NMDG) was lower than the G(Na) and the replacement of Na(+) with NMDG decreased the G(Cl). The G(gluconate) was lower than the G(Cl) and the replacement of Cl(-) with gluconate also decreased the G(Na). These observations suggest the interaction of cations and anions on paracellular ionic conductances; i.e., cations affect paracellular anion conductances and anions affect paracellular cation conductances.