1. The effect of tetraethylammonium ions (TEA) on potassium or rubidium permeability was studied in canine tracheal smooth muscle. 2. TEA (15-30 mM) markedly increased the rate of 42K- and 86Rb-efflux in normal Krebs solution. This increase is probably due mainly to the occurrence of electrical activity such as spike potentials and only partially to depolarization. 3. The rate coefficients of 42K- and 86Rb-efflux from depolarized tracheal smooth muscle bathed in a medium with elevated potassium (50-100 mM) were so large that the coefficients did not remain constant. When chloride ions in the medium were replaced with larger anions such as acetate, propionate or benzoate, the rate of 86Rb-efflux remained constant even in high-potassium solution (70 mM). 4. TEA caused a remarkable blockade of 86Rb-efflux in depolarized tracheal smooth muscle. The maximum decrease in the rate coefficient by TEA was approximately 52.5% of the control value in high-potassium-acetate solution. The dissociation constant for the interaction between TEA and its sites of action on the cell membrane was about 0.93 mM. 5. Spontaneous activity was, however, elicited only when TEA was added at a concentration of 10-30 mM and the membrane was depolarized more than 15-20 mV. It is assumed that TEA blocks not only the voltage-sensitive potassium conductance but also the conductance in the resting state, and that the latter may be possibly less sensitive to TEA.