The time course of two types of Ca(2+)-dependent currents were compared in single smooth muscle cells freshly isolated from guinea-pig trachea. When the pipette solution contained mainly 140 mM KCl, depolarization from -60 mV to 0 mV evoked an initial inward current followed by an outward current which consisted of transient (I(to)) and sustained components. In addition, a long-lasting inward tail current (Itail) was occasionally observed after the repolarization to -60 mV. Although I(to) often occurred repetitively during depolarization, the first I(to) reached the peak of approximately 50 ms after the start of depolarization and had the largest amplitude in most cells examined. The amplitude of Itail increased with the increase in depolarization period up to about 500 ms. Pharmacological analyses indicate that I(to) and Itail are Ca(2+)-dependent K+ and Cl- currents (IK-Ca and ICl-Ca), respectively, and suggest that not only Ca(2+)-influx through Ca2+ channels but also subsequent Ca2+ release from stores contributes to activate these currents. Spontaneous transient outward and inward currents, IK-Ca and ICl-Ca, respectively, were simultaneously recorded at -40 mV. In over 80% of the spontaneous current events, outward and inward currents coupled one to one and always occurred in this order. Puff-application of 10 mM caffeine also induced IK-Ca and ICl-Ca in this order at -40 mV. When caffeine was applied twice with various intervals, the current amplitude in the second application depended upon the period of the interval. The recovery of ICl-Ca during the interval was faster than that of IK-Ca. The results indicate that the activation and decay time courses of ICl-Ca are slower but its recovery is faster than those of IK-Ca.