The effects of lidocaine, tetracaine, procaine and bupivacaine (less than 1000 microM) on the Chara corallina internodal cell were studied. These local anesthetics depolarized the membrane at rest, while they affected the rising phase and the peak level of action potential not appreciably. Instead, they prolonged the time course of the falling phase of action potential as slowly as the repolarization was imperfect, even after enough lapse beyond the refractory period. Consequently, an action potential appeared to enhance the degree of depolarization at rest. Such a depolarization with stimulus/excitation was named use-dependent depolarization, while the depolarization without excitation, the resting one. The order of the potency of the use-dependent depolarization almost coincided with that of the nerve-blocking potency. During depolarization the change in membrane conductance was not simple. However, the conductance-voltage (Gm-Vm) relationship curve in the presence of local anesthetic suggested that depolarization was due to, not only the decrease in the electrogenic H(+)-pump, but also the increase in the diffusion conductance.