1. Whole-cell potassium currents were studied in single smooth muscle cells enzymatically isolated from pig coronary arteries. 2. In cells isolated from small diameter branches of the left anterior descending coronary artery (LAD), an inward rectifier potassium current (IK(IR)) was identified, which was inhibited by extracellular barium ions, suggesting the presence of inward rectifier potassium (KIR) channels. 3. The conductance for IK(IR) measured in 6, 12, 60 and 140 mM extracellular potassium was a function of membrane potential and the extracellular potassium concentration. 4. On hyperpolarization, IK(IR) activated along an exponential time course with a time constant that was voltage dependent. 5. Inward rectifier current was compared in cells isolated from coronary vessels taken from different points along the vascular tree. Current density was greater in cells isolated from small diameter coronary arteries; at -140 mV it was -20.5 +/- 4.4 pA pF-1 (n = 23) in 4th order branches of the LAD, but -0.8 +/- 0.2 pA pF-1 (n = 11) in the LAD itself. 6. In contrast to IK(IR), there was little effect of arterial diameter on the density of voltage-dependent potassium current; densities at +30 mV were 12.8 +/- 1.3 pA pF-1 (n = 19) in 4th order branches and 17.4 +/- 3.1 pA pF-1 (n = 11) in the LAD. 7. We conclude that KIR channels are present in pig coronary arteries, and that they are expressed at a higher density in small diameter arteries. The presence of an enhanced IK(IR) may have functional consequences for the regulation of cell membrane potential and tone in small coronary arteries.