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Effects of ethmozine (moricizine) on the rapid inward sodium current (INa) were studied in freshly isolated single cells of rat ventricular myocardium. INawas measured by means of a patch clamp method for observing integral ionic currents. Ethmozine was applied extracellularly to a small cell membrane patch at concentrations of 10, 20, and 40 μM. At a stimulation frequency of 0.1 Hz the drug decreased the peak INawithout producing a shift of the current–voltage curve, but shifted the V0.5of the steady-state inactivation curve by −6 mV. At frequencies of 2–5 Hz the ethmozine-induced block exhibited a prominent use dependence, with trains of depolarizing clamp pulses 5–50 ms in duration eliciting maximal INafrom holding potentials at which the steady-state inactivation variable h∞ was close to 1. The use-dependent inhibition of INabecame more pronounced with an increase in both stimulation rate and pulse duration. In contrast to what has been observed in the node of Ranvier of the frog, the present results indicate that ethmozine binds to both inactivated and open Nachannels, but that the contribution of the open channel block to the overall block at depolarizing clamp step durations of several hundred milliseconds is small in comparison with the contribution of the block of inactivated channels.