The unidirectional influxes of several monovalent ions into the epithelium ofin vitro rat jejunum were determined as a function of the transepithelial electrical potential difference using the technique described by Frizzell and Schultz (J. Gen. Physiol. 59:318, 1972). The results indicate that the passive conductance pathway accounts for approximately 80% of the total tissue conductance and is cation-selective. The relative ionic permeabilities of this pathway, which is in all likelihood largely if not entirely extracellular, isP ^K≧P ^Rb>P ^Na≫P ^Cl. The sequence of relative cation permeabilities corresponds most closely to Eisenman's sequence V suggesting that the route possesses a negative electrical field strength of intermediate intensity. The unidirectional influxes of lysine and tetraethylammonium (TEA) were unaffected over the range of ±50mV. These findings are consistent with the notion that the pasive conductance pathway is impermeable to solutes with an equivalent diameter greater than 8 Å; however, the possibility that factors other than ionic size are responsible for the exclusion of TEA and lysine from the shunt pathway cannot be excluded. The diffusional influx of Na closely agrees with the transepithelial serosa-to-mucosa flux of Na suggesting that the latter is largely, if not entirely, mediated by the passive conductance pathway and may circumvent the limiting membranes of the epithelial cells.