To compare the effect of halothane and enflurane on an atracurium-induced neuromuscular blockade, the authors studied 40 patients during elective surgery. During 1.25 MAC enflurane–nitrous oxide (n = 20) or halothane–nitrous oxide (n = 20) (MAC value includes contribution from 60% nitrous oxide), the doses depressing twitch tension 50% (ED50s) for atracurium were 70 and 77 μg/kg, respectively. The difference was not significant. Time from injection to peak effect did not differ between groups. However, the duration of action of atracurium (expressed as durations50or the duration of a 50% blockade) was longer during enflurane–nitrous oxide anesthesia (34.2 min) than during halothane–nitrous oxide anesthesia (25.5 min) (P< 0.05). The authors conclude that the potency of atracurium does not differ during halothane-nitrous oxide and enflurane-nitrous oxide anesthesia. Combining the results of this study with a previous study (atracurium ED50= 68 μg/kg and 83 μg/kg during isoflurane–nitrous oxide and fentanyl–nitrous oxide anesthesia respectively), the potency of atracurium does not differ by more than 20% among the four anesthetic techniques studied. The background anesthetic appears to have less effect on an atracurium-induced neuromuscular blockade than on one produced by other longer-acting nondcpo-larizing muscle relaxants (e.g., pancuronium andd-tubocurarine).