1. In order to evaluate the therapeutic effects of antihypertensive agents on malignant hypertension in M-SHRSP, EEG were performed as a non-invasive, therapeutic index. 2. Under pentobarbital anaesthesia, the EEG pattern of rats with severe hypertension and/or cerebrovascular lesions showed alternate short-active and long-depressive phases with spike and sharp waves. 3. When M-SHRSP were treated with an angiotensin converting enzyme inhibitor (SQ 29,852) or a calcium antagonist (manidipine), the depressive phases became shorter and active phases longer. These changes were more prominent in manidipine treated rats than in SQ 29,852 treated rats. 4. The EEG spike- and sharp-wave complex seems to be a convenient index for evaluating cerebrovascular lesions and cerebral activity in M-SHRSP.