The electrophysiological properties of acutely-isolated canine articular chondrocytes have been characterized using patch-clamp methods. The "steady-state" current-voltage relationship (I-V) of single chondrocytes over the range of potentials from -100 mV to +40 mV was highly non-linear, showing strong outward rectification positive to the zero-current potential. Currents activated at membrane potentials negative to -50 mV were time-independent, and the I-V from -100 mV to -60 mV was linear, corresponding to an apparent input resistance of 9.3 ± 1.4 G(n=23). The outwardly rectifying current was sensitive to the K⁺ channel blocking ion tetraethylammonium (TEA), which had a 50% blocking concentration of 0.66 mM (at +50 mV). The "TEA-sensitive" component of the outwardly rectifying current had time- and membrane potential-dependent properties, activated near -45 mV and was half-activated at -25 mV. The reversal potential of the "TEA-sensitive" current with external K⁺ concentration of 5 mM and internal concentration of 145 mM, was -84 mV, indicating that the current was primarily carried by K⁺ ions. The resting membrane potential of isolated chondrocytes (-38.1 ± 1.4 mV; n=19) was depolarized by 14.8 ± 0.9 mV by 25 mM TEA, which completely blocked the K⁺ current of these cells. These data suggest that this voltage-sensitive K⁺ channel has an important role in regulating the membrane potential of canine articular chondrocytes.