The 24-hr sleep-wake distribution and power spectra of the electroencephalogram were determined in rabbits that had been implanted with cortical and hippocampal electrodes. A diurnal preference for sleep was observed. The spectral power density in nonrapid eye movement sleep (NREM sleep) of the cortex showed a decreasing trend in most frequencies within the 12-hr light period. In the 12-hr dim period no clear trend was present. Most hippocampal EEG frequencies decreased in NREM sleep in the first two hours of the light period, and thereafter stayed on a constant level. Sleep deprivation elicited the following changes: a prolonged increase of NREM sleep and a short increase of REM sleep; in the cortex, an increase of slow-wave activity (SWA; power density in the 0.25-2.0 Hz frequency band) in NREM sleep, which declined in the course of recovery; an enhancement of slow-wave (1.25-3 Hz) and theta (6.25-7 Hz) activity in REM sleep. The hippocampus showed an increase in NREM sleep power density in almost all frequencies. In REM sleep the hippocampus exhibited an increase in power density in the 6.25-7 Hz and 12.25-13 Hz bands, whereas in the 7.25-8 Hz band the values were below baseline. The results show that SWA in NREM sleep and theta activity in REM sleep are enhanced by sleep deprivation, as has been observed in other mammalian species. The EEG changes in the hippocampus resembled those in the cortex.