Membrane potentials, action potentials and macroscopic currents in enzymatically dispersed, single smooth muscle cells of the circular layer of cat and rabbit colon were investigated. The cells did not exhibit spontaneous depolarizations and repolarizations (slow waves) or spontaneous action potentials. Single action potentials of smooth muscle cells were evoked by depolarizing current pulses of 5 ms to 3 s duration. A repetitive action potential discharge and an increase in the duration of the action potential was observed in cells during long depolarizing current pulses by superfusion with tetraethylammonium (TEA) or 4-aminopyridine (4-AP). Tetrodotoxin (TTX) did not alter the configuration of the action potential. Voltage-clamp experiments revealed two major outward macroscopic currents: a quasi-instantaneous (time-independent) and a time-dependent outward current. Both currents were identified as potassium (K) currents due to their pharmacological sensitivity to K antagonists [TEA, 4-AP and cesium (Cs)] and due to the reversal potential of outward tail currents. Barium selectively blocked the time-independent current. A time-dependent outward K current in colon cells was observed which appeared to be dependent upon entry of calcium ions (Ca2+) through voltage-dependent Ca-channels, since it was blocked by cadmium and low concentrations of nifedipine. The majority of cells did not exhibit transient outward currents. Inward currents were exposed in some of the cells when the K currents were blocked by external TEA and by replacement of K by Cs and TEA in the recording pipette. Inward currents were presumably carried by Ca2+, since they were not altered by TTX, were sensitive to external Ca concentrations and were abolished by the Ca channel antagonist, nifedipine. Carbachol augmented the amplitude of the inward Ca current.