Voltage-dependent potassium channels (Kv channels) have been reported to be involved in a variety of contractile responses of small arteries. In small arteries of the systemic circulation the expression of Kvα1 and Kvα2 subunits has been detected. However, due to a lack of specific inhibitors the contribution of these subunits to the Kv current is still unknown. Recently, a specific inhibitor of Kvα2 subunits, stromatoxin, was discovered. Thus, the hypothesis was tested that Kvα2 subunits are a major component of the Kv current in smooth muscle cells freshly isolated from Wistar rat posterior cerebral arteries employing the patch-clamp technique. Animals were humanely killed. An outward current with biophysical and pharmacological properties similar to recently described Kv currents of rat and mouse small arteries was found in the cells studied. Stromatoxin inhibited the Kv current in a concentration-dependent manner with an ED₅₀ of 36 nM and a maximum effect of 61.2%. At 100 nM stromatoxin produced a fast, reversible inhibition of the Kv current. The voltage dependence of activation of the Kv current in the presence of 100 nM stromatoxin was characterized by a potential of half-maximal activation of -6.2±2.3 mV (n=10; mean±S.E.M.) and a slope of 12.3±1.0 mV (n=10), which are not different from the potential of half maximal activation of -4.4±1.9 mV (n=10) and the slope of 12.1±0.9 mV (n=10) obtained in the absence of stromatoxin. The inactivation of the Kv current in the presence of 100 nM stromatoxin was characterized by a potential of half maximal inactivation of -45.1±2.5 mV (n=10), a slope of 7.7±0.7 mV (n=10) and a sustained current at maximal inactivation of 12±2% of the initial current (n=10). These parameters show some difference compared to the characteristics obtained in the absence of stromatoxin, where the potential of half maximal inactivation was -37.9±1.4 mV (n=10; p<0.05, t test), the slope was 9.3±0.5 mV (n=10) and the sustained current at maximal inactivation was 9±2% of the initial current (n=10). The recovery from inactivation in the presence of 100 nM stromatoxin had a time constant of 2.12±0.20 s (n=10), which is not different from the time constant for recovery from inactivation in the absence of stromatoxin of 2.14±0.24 s (n=10). Thus, the data show that a stromatoxin-sensitive current, most probably carried by Kvα2 subunits, contributes considerably to the Kv current of smooth muscle cells isolated from Wistar rat posterior cerebral arteries. The presence of the stromatoxin-sensitive current affects inactivation of the Kv current resulting in a larger current at physiological membrane potentials.