1. Electroencephalogram (EEG) of stroke-prone spontaneously hypertensive rats (SHRSP) and M(malignant)-SHRSP were analysed for excitability changes in the cerebral neuronal circuit using power spectral comparisons. 2. EEG in hypertensive SHRSP and M-SHRSP showed the emergence of slow wave bursts parallel to the development of hypertension. 3. These bursts were composed of spikes and sharp waves, and became dominant in the later stages of hypertension. 4. Bursts were interposed by a silent period indicating a strongly depressed state of neuronal activities. The ratio of the periods of the depressed to the burst increased with the progress of the hypertension. 5. A shift of the major component toward a lower frequency band also indicated decreased cerebral activity.