A double-blind, randomized controlled trial using an electroencephalograph computerized analysis and cartography was carried out to investigate the spectral modifications induced by diazepam and hydroxyzine. Without monitoring response to stimulation, the spectra found for diazepam and for hydroxyzine were qualitatively very similar, showing increase of the slow waves, reduction of the alpha rhythm and accentuation of the beta 1 rhythms. These traces suggested strongly that both drugs had produced a sedative, anti-anxiety effect. The intensity of the effect produced by 50 mg of hydroxyzine appeared to be less than that produced by 10 mg diazepam. After monitoring response to stimulation, the spectra were modified and the reactivity of the two drugs differed with regard to the slow delta, theta and alpha 1 frequency bands. It was possible to distinguish between the sedative and anti-anxiety effects of both diazepam and hydroxyzine. Even if the two drugs had some similar effects, the mode of action in the central nervous system was certainly different, as can be seen from the characteristics of distribution of the slow waves, their reactivity and, with regard to frequency, the fluctuation of the dominant frequency of rapid rhythms.