Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.