Administration of a subparalyzing dose of nondepolarizing muscle relaxant (priming dose) prior to its intubating dose hastens the onset time (time from muscle relaxant administration to 100% depression of twitch tension) of neuromuscular blockade. This study was undertaken to determine the optimal priming and intubating doses and time interval between these doses (priming interval) of vecuronium during rapid-sequence induction of anesthesia. The authors measured single-twitch tension in 79 healthy, awake, premedicated (fentanyl, 50–150 mg iv, and/or diazepam, 5–10 mg iv) patients. In Part A of the study, the priming dose was varied (0.0, 0.005, 0.01,0.0015, or 0.02 mg/kg iv). Decrement of twitch tension and symptoms were recorded 3 min later. Four minutes after the priming dose, thiopental, 4–6 mg/kg iv, and vecuronium, 0.1 mg/kg iv, were given. Onset times for the 0.01,0.015, and 0.02 mg/kg groups were significantly shorter than for 0.005 and 0.0 mg/kg groups. No breathing difficulties were encountered in any of the groups. Decrement of twitch tension greater than 25% of control only occurred in the 0.02 mg/kg group (4 of 11 patients). In Part B, the priming interval was varied (2, 4, or 6 min) after giving the optimal priming dose (0.01 mg/kg). Anesthesia was induced as in Part A. Onset times for the 4-min group were significantly faster than the 2− or 6-min groups. In Part C, the intubating dose was varied (0.07, 0.1, or 0.15 mg/kg iv) after the optimal priming dose and optimal priming interval (4 min). Onset times for the 0.1 mg/ kg and 0.15 mg/kg groups were significantly faster than the 0.07 mg/kg group. Intubating conditions were not evaluated. The authors conclude that when vecuronium is used for rapid establishment of neuromuscular blockade during induction of anesthesia, 0.01 mg/ kg iv should be given 4 min prior to 0.1 mg/kg iv.