Patch-clamp techniques were used to study isolated renal cortical collecting ducts of rabbits. Gigaohm seals of the native apical membranes of the principal cells were obtained from tissues superfused with a Ringer solution. No enzymatic or other pretreatment of the tissues was required. The patches studied were primarily of the on-cell type, although excised patches could be obtained. Unitary currents in a range of tenths of picoamperes were observed at holding voltages between +/- 100 mV. Since the apparent reversal potential was at a holding voltage at or near 0 eatment of the tissues was required. The patches studied were primarily of the on-cell type, although excised patches could be obtained. Unitary currents in a range of tenths of picoamperes were observed at holding voltages between +/- 100 mV. Since the apparent reversal potential was at a holding voltage at or near 0 eatment of the tissues was required. The patches studied were primarily of the on-cell type, although excised patches could be obtained. Unitary currents in a range of tenths of picoamperes were observed at holding voltages between +/- 100 mV. Since the apparent reversal potential was at a holding voltage at or near 0 mV and since the current-voltage relationship was markedly nonlinear, the unitary currents are most likely due to K+ . Na+-channel current fluctuations, if present, could not be uniquely identified in the presence or absence of amiloride.