Ionic conductances of rabbit osteoclasts were investigated using both whole-cell and cell-attached configurations of the patch-clamp recording technique. The predominant conductance found in these cells was an inwardly rectifying K+ conductance. Whole-cell currents showed an N-shaped current-voltage (I-V) relation with inward current activated at potentials negative to EK. When external K+ was varied, I-V curves shifted 53 mV/10-fold change in [K+]out, as predicted for a K(+)-selective channel. Inward current was blocked by Ba2+ and showed a time-dependent decline at negative potentials, which was reduced in Na(+)-free external solution. Inward single-channel currents were recorded in the cell-attached configuration. Single-channel currents were identified as inward-rectifier K+ channels based on the following observations: (i) Unitary I-V relations rectified, with only inward current resolved. (ii) Unitary conductance (gamma) was 31 pS when recorded in the cell-attached configuration with 140 mM K+ in the pipette and was found to be dependent on [K+]. (iii) Addition of Ba2+ to the pipette solution abolished single-channel events. We conclude that rabbit osteoclasts possess inwardly rectifying K+ channels which give rise to the inward current recorded at negative potentials in the whole-cell configuration. This inwardly rectifying K+ current may be responsible for setting the resting membrane potential and for dissipating electrical potential differences which arise from electrogenic transport of protons across the osteoclast ruffled border.