The purpose of this study was to determine whether prior administration of a small, sub-paralytic dose of nondepolarizing muscle relaxant, vecuronium, would have a better intubating condition than a single bolus dose when vecuronium was used as the muscle relaxant during rapid sequence induction and tracheal intubation. Six groups of 15 patients each (ASA class I or II) were involved in this study. Anesthesia was induced with fentanyl 2-3 micrograms/kg and thiopental 4-5 mg/kg. In group 1, 2 and 3, the patients were given the same priming dose of 0.1 mg/kg. Four min later, different doses of vecuronium were given to each group: 0.09 mg/kg (Gp1), 0.14 mg/kg (Gp2), and 0.19 mg/kg (Gp3). In group 4, 5 and 6, no priming vecuronium was given and the intubating doses given to each group were as follow: 0.1 mg/kg (Gp4), 0.15 mg/kg (Gp5) and 0.2 mg/kg (Gp6). At the end of a 60 sec, the percentage of patient with excellent intubating condition were 46.6%, 66.6%, 86.6%, 20%, 40% and 80% respectively for group 1, 2, 3, 4, 5 and 6 respectively. The percentage with good intubating condition were 46.6%, 33.4%, 13.4%, 46.6%, 53.2%, and 20% for group 1, 2, 3, 4, 5 and 6 respectively. The percentage of poor intubating condition were 6.8% in group 1, 33.4% in group 4, 6.8% in group 5 and 0% in group 2, 3, 6. All patients receiving a priming dose could sustain head lift for 5 sec and did not complain any respiratory discomfort 3 min after the priming dose.(ABSTRACT TRUNCATED AT 250 WORDS)