The mechanism of potentiation of mechanical activity by tetraethylammonium chloride (TEA) was investigated in canine tracheal smooth muscle. Twitch tension induced by direct electrical stimulation was potentiated by 1 mM TEA to more than twice the amplitude. Although the tracheal strips usually exhibited no myogenic response to quick stretch, application of TEA (greater than or equal to 10 mM) produced myogenic responses to quick stretch. This response, however, could be induced in the presence of less than 3 mM TEA in a solution containing 10.9 or 15.9 mM potassium. TEA (2 mM) significantly potentiated the contraction induced by 20 mM potassium, and the potentiation was not observed in the absence of calcium ion. On the other hand, 2 mM TEA did not potentiate the contraction induced by CaCl2 in strongly depolarized muscle. Tetramethyl- (TMA) and tetrapropylammonium (TPA) did not produce myogenic responses to quick stretch, and there was no potentiating effect on the responses to direct electrical stimulation or to high levels of potassium. The blocking activity of these two ammonium ions on 86Rb-efflux was considerably less than seen with TEA. In conclusion, the blocking of potassium conductance by TEA, even at concentrations lower than 5 mM, results in the increase in Ca conductance when electrically, mechanically or chemically stimulated. TEA probably increased the transmembrane Ca-influx as a result of the potentiation of cell membrane depolarization.