Influence of the severity of myasthenia gravis (MG) on endplate potential (EPP). Amplitude of the first EPP (EPP₁amp; A) and the quotient between the averaged amplitude from the 21st to the 50th EPPs (EPP_21–50amp) and EPP₁amp, as an indication of EPP run-down (B), were significantly smaller at 0% inhibitory concentration (IC0) for first evoked isometric twitch tension of rocuronium in the moderate and severe MG groups than in the sham group. Quantal content of EPP₁ (QC₁; C) and EPP_21–50 (QC_21–50; D) were significantly different at IC0 of rocuronium among the three groups. EPP₁amp (A) was significantly decreased by administration of IC50 of rocuronium in the three groups (A), whereas indicators of EPP run-down (B), QC₁ (C), and QC_21–50 (D) were significantly decreased in the sham and moderate MG groups by administration of IC50 of rocuronium. Data are shown as means ± SD, n = 8 in each group. **P < 0.01 versus sham group at IC0 of rocuronium, ††P < 0.01 versus moderate MG group at IC0 of rocuronium, §§P < 0.01 versus each group at IC0 of rocuronium. Data were analyzed by using two-way repeated-measures ANOVA with the Tukey test among three groups and two-tailed paired t test with Bonferroni correction between IC0 and IC50 in each group.