BACKGROUND: The bispectral index (BIS) has been used as an indicator of a sedative state and has been considered to be related to anesthetic agents and noxious stimulus. In this study, we measured the BIS, blood pressure (BP) and heart rate (HR) during induction of anesthesia after premedication with or without midazolam or morphine and evaluated the bispectral index as an indicator of an objective evaluation of midazolam premedication and relation to the cardiovascular response to the anesthetic induction. METHODS: Seventy five patients scheduled to undergo elective surgery under general anesthesia were divided into 3 groups. Each group received midazolam and glycopyrrolate (midazolam group, n = 25), or morphine and glycopyrrolate (morphine group, n = 25), or glycopyrrolate only (control group, n = 25) as premedication (midazolam 0.08 mg/kg IM, morphine 0.05 mg/kg IM, glycopyrrorate 0.2 mg IM). Then, anesthetic induction (propofol 2 mg/kg, succynylcholine 1 mg/kg) was done. The bispectral index, blood pressure, and heart rate were measured at before induction, after propofol injection, and intubation. RESULTS: The Bispectral index was significantly lower in the midazolam group and the morphine group compared with the control group before anesthetic induction. Blood pressure was not significantly different among the three groups. Heart rate was significantly lower in the midazolam group compared with the control group before anesthetic induction. CONCLSIONS: Midazolam or morphine premedicated patients appear to maintain a stable heart rate and have a low BIS at before induction. The Bispectral index could be objectively used in midazolam-premedicated patients when evaluating the degree of sedation and predicting hemodynamic changes, and probably in morphine-premedicated patients also.