The intravenous injection of rocuronium bromide is often painful, and different methods have been used to minimize the incidence and severity of this pain. This study determined the effective dose of alfentanil to minimize the injection pain of rocuronium and the cardiovascular response after endotracheal intubation. Methods: Eighty ASA physical status 1 and 2 adult patients were divided into four groups. Sixty seconds before administering rocuronium 0.6 mg/kg, the groups were given 10 ml of intravenous normal saline or alfentanil 10, 15, and 20μg/kg. Pain was assessed after rocuronium injection. The mean arterial pressure and heart rate were measured before induction and before and after intubation. Results: Both 15 and 20μg/kg alfentanil minimized the rocuronium injection pain, although 20μg/kg alfentanil caused an undesirable decrease in the mean arterial pressure. Conclusions: A 15μg/kg bolus of alfentanil may be useful for minimizing the rocuronium injection pain