BACKGROUND Succinylcholine has been the agent of choice when clinical conditions require emergency airway protection during a rapid-sequence induction of anesthesia. Rocuronium, a new nondepolarizing muscle relaxant with a brief onset of action, but devoid of the adverse reactions associated with succinylcholine, may be an alternative to succinylcholine. To test this hypothesis, the authors compared rocuronium with succinylcholine and vecuronium for rapid-sequence induction of anesthesia. METHODS Fifty patients, ASA 1-3, were randomly designated to receive one of three intravenous doses of rocuronium (0.6, 0.9, and 1.2 mg/kg), vecuronium (0.1 mg/kg), or succinylcholine (1.0 mg/kg). Patients were premedicated with midazolam and fentanyl, and received 2-7 mg/kg thiopental for induction of anesthesia. Sixty seconds after receiving a muscle relaxant, intubation of the trachea was attempted by a clinician who was blinded to the muscle relaxant administered. Neuromuscular monitoring was established before administration of the muscle relaxant. The time from injection of muscle relaxant until complete ablation of T1 (onset) and recovery of T1 to 25% (duration) were recorded. Tracheal intubating conditions were evaluated, and the presence or absence of fasciculations was noted. RESULTS Onset times for patients receiving 0.9 mg/kg (75 +/- 28 s) and 1.2 mg/kg rocuronium (55 +/- 14 s), and succinylcholine (50 +/- 17 s) were similar. Onset times for the groups given 0.6 mg/kg rocuronium (89 +/- 33 s) and vecuronium (144 +/- 39 s) were significantly longer. Clinical duration of action was longest with 1.2 mg/kg rocuronium, similar with 0.6 and 0.9 mg/kg rocuronium, and vecuronium, and least with succinylcholine. CONCLUSIONS There is a dose-dependent decrease in onset time with rocuronium. The onset times for the two larger doses of rocuronium were similar to that for succinylcholine, but clinical duration of action with rocuronium was significantly longer. The brief onset time achieved with rocuronium indicates that administration of 0.9-1.2 mg/kg is an acceptable alternative to succinylcholine for rapid-sequence induction of anesthesia.