We investigated the interaction between mivacurium and succinylcholine when mivacurium was administered during the early recovery from succinylcholine block. We studied 40 adult patients during propofol-alfentanil-N2O-O2 anesthesia. Neuromuscular function was monitored using an electromyographic method (Relaxograph, Datex, Helsinki, Finland). Patients randomly received either 1.0 mg/kg of succinyl-choline followed by 0.15 mg/kg of mivacurium when the first twitch (T1) during succinylcholine block recovered to 5%, or 0.15 mg/kg of mivacurium without succinylcholine. Serum cholinesterase activity was lower than normal range in two patients and higher than normal range in four patients, but the dibucaine number value was normal in every patient. The mean onset time (3.8 +/- 0.9 min) (mean +/- SD) or maximal neuromuscular block (96.6% +/- 7.2%) of mivacurium did not differ between the groups. The T1 recovery times of mivacurium were slightly shorter (P < 0.05) after succinylcholine administration than without it. During recovery of mivacurium block, the fade was significantly greater, i.e., the train-of-four (TOF) ratio was lower, after succinylcholine administration than without it. Recovery index (T1 25%-75%, mean 4.7 +/- 1.3 min) and the time from the administration of mivacurium to the recovery of TOF ratio 0.7 (mean 20.4 +/- 5.1 min) were not different between the groups. In conclusion, in healthy patients succinylcholine has negligible effects on a subsequent mivacurium-induced neuromuscular block.