We have investigated the two types of voltage-dependent outward potassium (K) currents, i.e. delayed rectifier K current (IK(V)) and 'A-like' transient outward K current (Ito) with patch-clamp technique in single smooth muscle cells (SMCs) isolated from rabbit basilar artery, and investigated the characteristics of them. The time-courses of activation were well fitted by exponential function raised to second power (n2) in IK(v) and fourth power (n4) in Ito. The activation, inactivation and recovery time courses of Ito were much faster than that of IK(V). The steady-state activation and inactivation of IK(V) was at the more hyperpolarized range than that of Ito contrary to the reports in other vascular SMCs. Tetraethylammonium chloride (TEA; 10 mM) markedly inhibited IK(V) but little affected Ito. 4-Aminopyridine (4-AP) had similar inhibitory potency on both currents. While a low concentration of Cd2+ (0.5 mM) shifted the current- voltage relationship of Ito to the positive direction without change of maximum conductance, Cd2+ did not cause any appreciable change for IK(V).