BACKGROUND: In intravenous administration of a depolarizing neuromuscular blocker, succinylcholine is reported to produce activation of the electroencephalogram and increase cerebral blood flow and intracranial pressure. In this point, rocuronium was recently introduced as a non-depolarizing relaxant, and recommended as a safe alternative to succinylcholine. The purpose of this study was to evaluate the effects of rocuronium and succinylcholine on cerebral blood flow velocities during anesthetic induction. METHODS: Forty patients were randomly assigned into two groups. Group 1 was administrated rocuronium 0.6 mg/kg and group 2 was administrated succinylcholine 1 mg/kg for tracheal intubation after each group had intravenous administration of thiopental 5 mg/kg. The author observed changes of mean arterial pressure, arterial carbon dioxide tension, and middle cerebral arterial blood flow velocities at 5 times: before induction (control), 30 sec after thiopental administration, 30 sec, 60 sec and 90 sec after muscle relaxant administration. RESULTS: Mean arterial pressure decreased more at 30 sec after thiopental administration compared with the control (P<0.05). Middle cerebral arterial blood flow velocities were reduced at 30 sec after thiopental administration and 60 sec after rocuronium administration compared with the control (P<0.05). Middle cerebral arterial blood flow velocities were reduced at 30, 60, 90 sec after rocuronium administration compared with succinylcholine administration (P<0.05). CONCLUSIONS: We conclude that rocuronium has little effects on increasing cerebral blood flow. These result suggest that rocuronium have a less effect on increase in cerebral blood flow during neurosurgical anesthesia.