BACKGROUND: The administration of rocuronium is associated with severe burning pain on injection that lasts for approximately 10-20 seconds. Injection pain is probably caused by the acidic pH of rocuronium. Mixing rocuronium with 8.4% sodium bicarbonate might neutralize the acidic pH thereby decrease the level of injection pain. This study investigated the appropriate sodium bicarbonate dose for preventing injection pain. METHODS: The study examined 250 patients (aged 20 to 60 years) from ASA I and II groups who scheduled for elective surgery. The patients were divided randomly into five groups. The control group (SB0) received rocuronium 50 mg (5 ml) only and the experimental groups received rocuronium 50 mg mixed with 8.4% sodium bicarbonate 1 (SB1), 2.5 (SB2.5), 5 (SB5), 7 (SB7) ml, respectively. The level of pain was evaluated as the withdrawal response as follows: no movement, 0; hand and wrist movement, 1 point; ipsilateral arm movement, 2 points; and general movement 3 points. RESULTS: The incidence of a withdrawal response was 68% in the control group (SB0). On the other hand, the incidence of a withdrawal response was 38%, 28%, 14% and 12% in the SB1, SB2.5, SB5 and SB7 groups, respectively. The withdrawal response was significantly lower in the experimental groups than in the control group (P < 0.01). In the experimental groups, a significant difference was observed between the SB1 and SB5, SB7 groups. However, there was no significant difference observed between the SB2.5, SB5 and SB7 groups. CONCLUSIONS: Mixing 5 ml of 8.4% sodium bicarbonate with rocuronium 50 mg (5 ml) is the most effective in preventing the injection pain associated with rocuronium during the induction of anesthesia.