Age-related spontaneous cortical and hippocampal EEG changes associated with level of arousal in rats were investigated. The EEG of rats with chronically implanted electrodes were recorded using bipolar leads and simultaneously analyzed with a computer to obtain the power spectra. In the awake stage, EEGs in the aged rats consisted predominantly of slow waves when compared with those in the young rats. The cortical EEG activity of the aged rats exhibited specific irregular burst waves which consisted of two spectral peaks at 8-9 Hz and 15-16 Hz during the drowsy period. The relative powers of the cortical and hippocampal waves in the aged rats differed from those of the young rats during the slow-wave sleep stage. These findings suggest that the slowing of the EEG in the rats during the awake stage may be related to decreased brain activity associated with aging, and that the irregular burst waves in the cortical EEG in the aged rats appear to correlate with the changes observed in age-related human sleep patterns.