The purpose of the present study was to determine the effects of magnesium sulfate (MgSO(4)) on the neuromuscular function and spontaneous breathing of patients under sevoflurane and spinal anesthesia. Twenty-two patients with a history of arrhythmia undergoing elective knee surgery were randomly assigned to two groups: group M (n = 11), administered with MgSO(4) 40 mg.kg(-1), and group S (n = 11), administered with saline. A combination of spinal anesthesia with 2% sevoflurane inhalation was applied to all patients under spontaneous breathing. Tidal volume (VT: ), respiratory rate (RR) and end-tidal carbon dioxide (ET(CO) (2)) were measured before the MgSO(4) or saline injection and measurements were repeated at 5, 15, 30, and 45 min after the injection. Neuromuscular function was continuously monitored with an acceleromyograph to record the acceleration of the adductor pollicis by stimulating the ulnar nerve at a frequency of 0.1 Hz. The VT: , RR, and ET(CO) (2) showed little change in either group, and there was no significant difference between, the groups. The single-twitch response showed significant differences between the two groups (P = 0.0006). The present study indicated that the MgSO(4) had a minimal effect on spontaneous breathing in patients undergoing sevoflurane and spinal anaesthesia, but that it attenuated the safety margin of neuromuscular function.