We have previously shown that in the presence of tetraethylammonium (TEA, 6.7-67 mM) phasic mechanical activity and a myogenic response (MR) to quick stretch are produced in normally multi-unit tracheal smooth muscle. The present studies were designed to investigate the electrophysiological basis for these changes in the mechanical properties of the muscle. Intracellular recordings showed that in the presence of TEA the membrane was partially depolarized and trains of small (8-20 mV), decrementally conducted action potentials were produced spontaneously at a frequency of 15-20/min. Action potentials could also be stimulated by external electrodes, and the conduction velocity over short distances was 0.84 plus or minus 0.2 cm/s. Membrane conductance and rectification, as measured by the magnitude of electrotonic potentials in response to external stimulation, were reduced in the presence of TEA. The length constant was increased from 1.6 plus or minus 0.1 to 2.8 plus or minus 0.2 mm. These results are consistent with the notion that TEA produces phasic membrane electrical activity by reducing P-K.