To determine the effect of the commonly used volatile anesthetics on a vecuronium-induced neuromuscular blockade, the authors studied 54 patients anesthetized with 1.2 MAC or 2.2 MAC enflurane, isoflurane, or halothane (MAC value includes contribution from 60% nitrous oxide). During 1.2 MAC enflurane, isoflurane, and halothane, the ED50s (the doses depressing twitch tension 50%) for vecuronium were 12.8, 14.7, and 16.9 μg/kg, respectively. During 2.2 MAC enflurane, isoflurane, and halothane, the ED50s for vecuronium were 6.3, 9.8, and 13.8 μg/kg, respectively (P< 0.05). Time from injection to peak effect was the same for each anesthetic group (6.5 ± 0.5 min, mean ± SD), except for the group given 2.2 MAC enflurane (9.7 ± 0.6 min) (P< 0.05). The duration of a 50% block from injection to 90% recovery was the same for each group (mean 20 ± 4 min), except for the group given 2.2 MAC enflurane (46.5 min) (P< 0.05). The authors conclude that enflurane is the most potent volatile anesthetic, followed by isoflurane and then halothane, in augmenting a vecuronium-induced neuromuscular blockade. Increasing the concentration of volatile anesthetic has less effect on a neuromuscular blockade produced by vecuronium than on one produced by other nondepolarizing relaxants (e.g., pancuronium andd-tubucurarine).