BACKGROUND: The "timing principle" utilises a single bolus of nondepolarizing neuromuscular blocking drug followed by an induction drug at the onset of clinical weakness. The purpose of this study was to compare the intubating conditions after succinylcholine or rocuronium and after rocuronium using the timing principle. METHODS: Forty patients were randomly allocated into four groups. Patients in group I received rocuronium 0.6 mg/kg using the timing principle. At the onset of clinical weakness, anesthesia was induced with the thiopental 4-5 mg/kg. Patients in group II, III, and IV received rocuronium 0.6, 0.9 mg/kg, and succinylcholine 1.5 mg/kg respectively using the usual technique. The trachea were intubated 60 s after thiopental induction. Accelerographic response to train-of-four (TOF) stimulation of the ulnar nerve was used for neuromuscular monitoring. Intubating conditions were assessed according to a grading scale. RESULTS: The twitch depression immediately before tracheal intubation in group I, II, III, and IV were 14.5, 28.2, 11.1, and 6.8%, respectively. The TOF count showed no significant differences between groups. The duration of action in group III (45.3 +/- 12.1 min) was significantly prolonged compared to that in group I (31.2 +/- 6.4 min). Intubation conditions were either good or exellent in all patients except one in group II. In group I, three patients recalled shortness of breath or general weakness. CONCLUSIONS: It is concluded that the use of rocuronium 2 X ED95 using the timing principle did not provide additional benefits compared to rocuronium 3 X ED95 using the usual technique except in duration.