The characteristics of phase II neuromuscular block following repeated intravenous injections of succinylcholine (SCh) were determined in 15 adult patients during enflurane, halothane, or fentanyl-nitrous oxide anesthesia. The onset of phase II block (train-of-four ratio (T4) equal to 0.5) occurred following a cumulative SCh dose of 4.4 +/- 0.3 (SEM) mg/kg of enflurane, 5.1 +/- 0.5 mg/kg of halothane, or 6.4 +/- 0.5 mg/kg of fentanyl. The cumulative SCh dose producing phase II block during fentanyl-nitrous oxide anesthesia was significantly greater than during enflurane (p less than 0.01) or halothane (p less than 0.05) anesthesia. Based on these data it is predicted that the likelihood of phase II block developing is less during fentanyl administration than during enflurane or halothane anesthesia. An abrupt transition from phase I to phase II neuromuscular block, as evidenced by an accelerated decrease in the T4 ratio, was observed during enflurane or halothane anesthesia as the cumulative SCh dose approached 3 to 5 mg/kg. The transition phase was delayed during fentanyl-nitrous oxide anesthesia, occurring after a cumulative SCh dose of 4 to 7 mg/kg. Following this transition, tachyphylaxis (decreased time between SCh injections) was observed in each study group. The T4 ratio in all three study groups stabilized at 0.15 to 0.25 after 7 to 8 mg/kg.