BACKGROUND Rocuronium, a new nondepolarizing muscle relaxant, has a rapid onset of activity and may be suitable as a component of a rapid-sequence induction of anesthesia. We evaluated a range of doses on onset and duration of effect at the larynx and the adductor pollicis and compared these characteristics with those of succinylcholine. METHODS Forty-eight patients aged 18-70 yr, of ASA physical status 1-3, were randomly allocated to receive succinylcholine (1 mg/kg) or one of three doses of rocuronium (0.4, 0.8, or 1.2 mg/kg) during surgery. Anesthesia was induced and maintained with propofol and fentanyl. The trachea was intubated without the use of muscle relaxants, and the cuff of the endotracheal tube placed between the vocal cords. Neuromuscular transmission was monitored by mechanomyography at the laryngeal adductor and adductor pollicis muscles. Muscular activity was evoked with supramaximal stimuli in a train-of-four sequence every 12 s to the anterior branch of the recurrent laryngeal nerve and the ulner nerve. RESULTS At the laryngeal adductors, peak effect exceeded 99% in all patients given succinylcholine and in none (0%), five (42%), and ten (83%) of those given rocuronium 0.4, 0.8, and 1.2 mg/kg, respectively. At the adductor pollicis, peak effect exceeded 99% in all study patients except two who received rocuronium 0.4 mg/kg (peak effects 91% and 97%). Onset of effect with succinylcholine was significantly more rapid at the laryngeal adductors (34 +/- 12 s, mean +/- SD) than at the adductor pollicis (56 +/- 15 s); this was true also for rocuronium 0.4 mg/kg (92 +/- 29 s and 155 +/- 40 s for the laryngeal adductors and adductor pollicis, respectively). Onset times were similar at the two muscle groups with rocuronium 0.8 and 1.2 mg.kg-1: 96 +/- 29 and 74 +/- 36 s with 0.8 mg/kg and 54 +/- 30 and 65 +/- 21 s with 1.2 mg/kg at the laryngeal adductors and the adductor pollicis, respectively. CONCLUSIONS The laryngeal adductors are more resistant to the action of rocuronium than is the adductor pollicis. Consequently, the onset of effect of rocuronium, in doses greater than 0.8 mg/kg, is similar to that of succinylcholine at the adductor pollicis but is significantly delayed compared with that of succinylcholine at the laryngeal adductors.