The efficacy of anaesthetic premedication has been assessed using sedative scores or a visual analogue scale. However, in both it may be difficult to exclude evaluators’ subjectivity or a placebo effect. Plasma concentration of catecholamines may also be useful for the assessment of patient anxiety. Recently bispectral electro-encephalographic analysis has been developed, and the bispectral index monitor has been reported to give measurements which correlate well with the depth of sedation. In the present study, we have examined the relation between bispectral index values and plasma catecholamine concentrations after oral diazepam premedication. Twenty-eight patients scheduled for elective surgery were randomly assigned to one of two groups: diazepam premedication group (group D(+), n=14) and no premedication group (group D(−), n=14). The patients were premedicated orally with diazepam 10 mg and roxatidine 75 mg in group D(+), and with roxatidine 75 mg only in group D(−) 90 min before arrival in the operating theatre. After patients arrived in the operating theatre, the bispectral index monitor was applied. Venous blood samples(6 mL) were collected in the case of patients in group D(+) for the measurement of plasma catecholamines levels using high-performance liquid chromatography. The bispectral index level (mean±SD) in group D(+): 93.5±773.5 was significantly lower than that in group D(−): 96.1±1.8(P<0.05). There was a significant correlation between bispectral index and plasma norepinephrine levels (r=0.567, P<0.05). The present study suggests that the bispectral index monitor may detect the effect of oral diazepam premedication.