Bilateral, electrolytic lesion of the lateral hypothalamus (LH) in rats produces hyposomnia and qualitative EEG changes which are difficult to assess by convencional visual inspections of electroencephalograms. In the present study the spectral analysis of EEG was applied in LH-lesioned rats and confronted with standart visual scoring method. One-hour samples of hippocampal and cortical EEG were taken from the light part of the circadian cycle before and after electrolytic or sham LH damage. In half of the LH-lesioned rats a power spectral analysis was performed using a Fast Fourier Transform routine at 1 Hz bands from 0.5 to 25 Hz; in the other half, as well as in the sham-lesioned group, EEG records were visually scored for the amount of waking, slow wave sleep and paradoxical sleep. Significant hyposomnia effects were found in LH-lesioned rats. Power spectral analysis of hippocampal EEG revealed a significant increase in power density at 4-6 Hz and reduction at 7-10, 14-17, 19-22 and 23-24 Hz. In neocortical EEG there was a significant increase in power density at 5-6 Hz band and reduction at 7-8 Hz. The results are discussed in the context of the effects of selective desrtuction of the specific neurotransmitter systems occupying the LH area.