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There have been no evidence-based comparisons of motor-evoked potential (MEP) monitoring with no and partial neuromuscular block (NMB). We compared the effects of different levels of NMB including no NMB on MEP parameters.MEP-monitored 120 patients undergoing neurosurgery were enrolled. The patients were randomly allocated to four groups: Group A was to maintain two train-of-four (TOF) counts; Group B was to maintain a T1/Tc of 0.5; Group C was to maintain a T2/Tc of 0.5 (T1,2, first or second twitch height of TOF; Tc, control twitch height); Group D did not maintain NMB. The mean MEP amplitude, coefficient of variation (CV), the incidence of spontaneous respiration or movement, the efficacy of MEP, and haemodynamic parameters were compared.The median [inter-quartile range (IQR)] amplitudes of the left leg for Groups A, B, C, and D were 0.23 (0.15–0.57), 0.44 (0.19–0.79), 0.28 (0.15–0.75), and 0.75 (0.39–1.35) mV, respectively. The median (IQR) CVs of the left leg were 71.1 (56.9–88.8), 76.1 (54.2–93.1), 59.8 (48.6–95.6), and 25.2 (17.3–35.0), respectively. The differences between groups of the mean amplitudes of the left arm and both legs were statistically significant (Kruskal–Wallis test, P=0.011 for the left leg). For all limbs, the differences between groups of the CVs were significant (P<0.001, for the left leg). Other parameters were not different.If NMB is used during MEP monitoring, a target T2/Tc of 0.5 is recommended. In terms of the MEP amplitude and variability, no NMB was more desirable than any level of partial NMB.