1. Two kinds of transient outward currents were observed upon depolarization of single smooth muscle cells isolated from guinea-pig ureter. The major transient outward current was through Ca2(+)-activated K+ channels (IK(Ca) which had a large conductance (130 pS; 126 mM [K+]i/5.9 mM [K+]o). 2. The smaller transient outward current (ITO) was pharmacologically separated from other membrane currents in the presence of 1 mM-Cd2+ and 2 mM-tetraethylammonium(TEA+) and was selectively blocked by 3 mM-4-aminopyridine. It peaked (approximately 200 pA) within 10 ms upon depolarization from -80 to +20 mV and its half-inactivation time was approximately 50 ms at +20 mV. Half-maximum voltages (V 1/2) for activation and inactivation were about -8 and -50 mV, respectively, in the presence of 1 mM-Cd2+ and 2 mM-TEA+. The time course of recovery from inactivation of ITO was fitted with a single-exponential function (tau = 100 ms at -80 mV). A tenfold change of [K+]o resulted in a 53 mV change in the reversal potential of the tail of ITO. 3. Cadmium reduced peak ITO and shifted the voltage dependence of activation and inactivation in the positive direction in a concentration-dependent manner. The V 1/2 for inactivation in the absence of Cd2+ was estimated to be approximately -64 mV. 4. Single-channel outward currents which appeared only in the initial part of a depolarizing pulse from about -100 mV were recorded using the cell-attached patch clamp. The decay of the ensemble average of the current was similar to the macroscopic ITO under whole-cell clamp. When the holding potential was less negative, the opening probability of the channel greatly decreased. The channel conductance in normal extracellular medium was 14 pS. 5. In ureter cells ITO resembles A-type current. ITO does not contribute significantly to the repolarization of the action potential but it may regulate membrane excitability by opposing Ca2+ current activated around the threshold of the action potential.