Tight junctions form the primary barrier regulating the diffusion of fluid, electrolytes and macromolecules through the paracellular pathway. Claudins are the major structural and functional components of tight junction strands and are considered as the best candidates for forming paracellular channels. They are a family of integral membrane proteins with more than 20 members and show distinct tissue distribution patterns. In this study, we found that claudin-7 is expressed in the distal and collecting tubules and the thick ascending limb of Henle of porcine and rat kidneys. To investigate the role of claudin-7 in paracellular transport, we have overexpressed a mouse claudin-7 construct in LLC-PK1 cells. Overexpression of claudin-7 did not affect the expression and localization of endogenous claudin-1, -3, -4, -7, and ZO-1. However, transepithelial electrical resistance in claudin-7-overexpressing cells was greatly increased. In addition, electrophysiological measurements revealed a dramatic reduction of dilution potentials in claudin-7-overexpressing cells compared to that of control cells. To determine which ions are responsible for the effects of claudin-7 overexpression on transepithelial electrical resistance and dilution potentials, we applied an ion substitution strategy. When NaCl was replaced with sodium aspartate, transepithelial electrical resistance was significantly decreased and dilution potentials were increased in claudin-7-overexpressing cells as compared to controls, the opposite effects from that of using NaCl. Furthermore, when NaCl was substituted by arginine-HCl or lysine-HCl, the increase in transepithelial electrical resistance was greater and the reduction in dilution potentials was smaller. Taken together, our studies demonstrated for the first time that the effect of claudin-7 overexpression in LLC-PK1 cells on paracellular transport is mediated through a concurrent decrease in the paracellular conductance to Cl(-) and an increase in the paracellular conductance to Na(+). These results support the model that claudin-7 may form a paracellular barrier to Cl(-) while acting as a paracellular channel to Na(+).