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1. Asymmetrical displacement currents ('gating currents') have been recorded in intracellularly perfused squid giant axons by averaging the currents associated with depolarizing and hyperpolarizing pulses. The relation between 'gating current' and Na inactivation was studied by investigating the effect of pulse duration and conditioning pulses. 2. Increasing the pulse duration from 0-3-1 msec to 10-20 msec reduced the off-response of the 'gating current' to 50-70% of its normal size; the time constant was 5 msec at +20 mV and 8 degrees C. The decrease of the Na current during a 10-20 msec pulse was stronger and faster; it decayed to 10-26% with a time constant of 1-35 msec. 3. The effect of pulse duration could also be demonstrated by using only depolarizing pulses. The charge displacement at the end of single or averaged depolarizing pulses was smaller for long pulse durations than for short. A long depolarizing pulse was followed by a small long-lasting tail of inward current. 4. A conditioning depolarizing pulse of 10-20 msec duration to a potential of -30 or +10 mV, followed by a short recovery period at -70 mV, decreased the on-response of the 'gating current'. Its size was reduced to 46-71% and 61-94%, respectively, for a recovery interval of 1-75 and 5 msec at 2-3 degrees C. The reduction of the Na current, measured under similar conditions, was more pronounced; the Na current was decreased to less than 50% of its normal value. 5. The observations about the effect of pulse duration and conditioning pulses on the 'gating current' are qualitatively consistent with those of Bezanilla & Armstrong (1974, 1975) and support the view that part of the asymmetrical charge displacement is inactivated during a 10-20 msec depolarization.

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