Using the patch clamp technique, one type of K+ channel was identified in the apical cell membrane of cultured principal cells of rabbit renal collecting ducts in the cell-attached or excised-patch configuration. The channel was highly selective for K+ over Na+ (typically 30-70-fold) and had a conductance of 180, SD +/- 39 pS (n = 6), referred to a situation of 140 mmolar K+-Ringer solution present on either surface of the patch membrane. Channel activity was completely blocked by Ba2+ (5 mmol/l) and partially inhibited by Na+. The latter was evidenced by a deviation from Goldman rectification at high cytoplasm-positive membrane potentials, which was observed when Na+ competed with K+ for channel entrance from the cytoplasmic surface. Channel open probability depended strongly on membrane voltage and cytoplasmic Ca2+ concentration. Open-close kinetics exhibited double exponential behaviour, with a strong voltage dependence of the slow open time constant. Infrequently also a substate conductance level was identified. The voltage and calcium dependence suggest that the channel plays a role in adjusting K+ secretion to Na+ absorption in the fine regulation of cation excretion in renal collecting ducts.