In this study, cerebral electrical activity or electro-encephalogram (EEG) was studied following exposure to high environmental heat, in three different age groups of freely moving rats. Each age group was subdivided into three groups: the acute heat stress group, subjected to a single exposure of 4h at 38°C in the biological oxygen demand incubator; the chronic heat stress group, exposed for 21 days, for 1 h each day, at 38°C in the incubator; and the handling control group. The polygraphic sleep-wake recordings involved simultaneous recordings of cortical EEG, electrooculogram (EOG), and electromyogram (EMG), on paper and in digital form on computer hard disk, just after the heat exposure for the acute stressed rats and on the 22nd day for the chronic stressed rats. The power spectrum was calculated for 2s epochs of the EEG signals. Quantitative analyses of EEG (qEEG) showed that, in all three age groups, changes in higher-frequency components (β2) were significant in all sleep-wake states following both acute and chronic heat stress conditions. The power of β2 activity in all three age groups after acute heat exposure was significantly decreased during slow wave sleep (SWS) (p<0.05) and rapid eye movement sleep (p<0.05), whereas the reverse was observed in the awake state (p<0.05). Following chronic heat exposure, β2 activity was found to increase in all three sleep-wake stages in all groups of rats (p<0.01 for SWS in the weaning group and p<0.05 for other data). Thus the study demonstrated that the cortical EEG is sensitive to environmental heat, and alterations in EEG frequencies in different states of mental consciousness due to high heat can be differentiated efficiently by EEG power spectrum analysis.