It has been reported that desensitization of the motor endplate, during prolonged application of acetylcholine, is associated with a marked shift in the 'reversal potential', interpreted as a change in ion selectivity of the permeable endplate channels. The matter has been re-examined by direct determination of reversal potentials, avoiding the linear extrapolation on which previous estimates were based. The present results show no significant alteration of the reversal potential ascribable to desensitization.