The EEG effects of 3 intravenous sedative drugs from different chemical families were studied during conscious sedation in 47 normal volunteers. The drugs studied were midazolam (a benzodiazepine), propofol (an alkylphenol) and thiopental (a barbiturate). Though these drugs cause different degrees of amnesia, they have the common EEG effects of suppressing α-rhythm and increasing total β-power. A large portion of the increase in β-power can be accounted for by β-rhythms. We used the UNIFAC-EEG technique to differentiate oscillatory systems underlying the rhythms induced by these drugs in a quantitative fashion. While thiopental induced β-rhythms which were similar to those appearing during drowsiness, midazolam and propofol induced β-rhythms with substantially different characteristics. The differences between the β-rhythms induced by drug infusion and previously described ‘sleep spindles’ are discussed. We conclude that a quantitative analysis of β-rhythms can differentiate the effects of these drugs