In vitro microperfusion was used to study the effect of low and high extracellular sodium concentrations on the transport capacity of the proximal convoluted tubule (PCT). Tubules from rabbit kidney were perfused with luminal and peritubular solutions containing 80, 115, 190, and 225 mM sodium. Control solutions contained 150 mM sodium. No ionic substitution was made and the proximal convoluted tubules were studied under hypo- or hypertonic conditions after a 20-min equilibration period. Sodium concentration was measured at 80, 150, and 225 mM sodium in the perfused and collected fluids and no significant difference was observed. Net sodium transport (JNa) remained relatively constant between 115 and 150 mM sodium. It decreased progressively at high sodium concentrations. Depression in JNa was also observed at 80 mM sodium. Fractional sodium reabsorption increased to 125.0% of control at 115 mM sodium and decreased to 69.4 and 40.1% at 190 and 225 mM sodium, respectively. At 80 mM, the results were not different from control. These findings indicate that at concentrations of 115-225 mM sodium the proximal convoluted tubule has the intrinsic capacity to regulate sodium transport.