Inside-out patch recordings from swine tracheal smooth muscle cells demonstrated the presence of four types of K+ channels (246, 140, 90 and 45 pS). The 246-pS K+ channel was most common. Both 246- and 90-pS channels are Ca++ sensitive, but the 90-pS channel was more sensitive at a membrane potential near the resting state. In whole cell recordings, spontaneous transient outward currents (STOCs) were observed. The STOCs arose from opening of Ca(++)-activated K+ channels. Removal of extracellular Ca++ did not eliminate STOCs. Decreasing intracellular free Ca++ with 10 mM EGTA markedly reduced STOC amplitudes. Single channel currents with a conductance similar to that of the large conductance Ca(++)-activated K+ channel were observed in whole cell recordings. The amplitude distribution for STOCs provides evidence that two populations of STOC events occur. The small STOC events were present at all membrane voltages. The large STOC events became more evident at more depolarized potentials (0 mV). We conclude that near the resting membrane potential, the small STOCs arise from opening of smaller conductance Ca(++)-activated K+ channels. As the cell is depolarized, the large STOCs arise due to opening of large conductance Ca(++)-activated K+ channels.