Confluent monolayers of the cultured renal distal tubule cell line (A6) were impaled with microelectrodes under short-circuit conditions. Specific membrane conductances were calculated from equivalent circuit equations. Transport properties of the apical and basolateral membranes were investigated during control conditions and short-term increases in basolateral potassium concentration [K +] from 2.5 to 20 mmol/1, with or without 0.5 mmol/1 Ba 2+ at the basolateral side. As in most other epithelia, the apical membrane represents the major resistive barrier. Transcellular, apical and basolateral membrane conductances (go, go and gi respectively), obtained from 22 acceptable microelectrode studies, averaged 61, 80 and 292 gS/cm 2, respectively. There was a highly significant correlation between short-circuit current (Isc) and go, whereas gi was unrelated to I~c. The /~c, which averaged 4.1 gA/cm 2, was almost completely blocked by amiloride. This was associated with fast hyperpolarization; the intracellular potential (V~o) increased from -69 to -83 mV and the fractional apical resistance rose to nearly 100%. Using the values of V~ during amiloride at normal and high [K+], an apparent transference number for K + at the basolateral membrane of 0.72 can be calculated. This value corresponds with the decrease in g~ to about 25% of the control values after blocking the K + channels with Ba 2 +. The nature of the remaining conductance is presently unclear. The cellular current decreased during high [K +] and Ba 2+, in part resulting from reduction of the electrochemical gradient for apical Na + uptake due to the depolarization. In addition, go decreased to less than 40%, which is considerably lower than predicted by the constant-field equation; this might indicate voltage sensitivity of the apical Na + permeability.