To clarify the preventive effects of ONO-3144, a free radical scavenger, on stroke-prone spontaneously hypertensive rats (SHRSP) and free radicals related to the hypertensive disorders. Drugs with powdered chow were administered orally to the rats from 2- to 14-month-old. Body weight, blood pressure, rectal temperature, oxygen consumption rate, thyroid hormones, lipids, platelet number, ocular fundus, autopsy, and life span were investigated. ONO-3144 did not affect the growth, blood pressure, concentrations of thyroid hormones and lipids in the blood, but decreased the rectal temperature and oxygen consumption rate. ONO-3144 also prevented the platelet number decrease, sclerotic change of retinal artery, edematous and hypertrophic changes in parenchymal and circulatory organs. Both the average life-span and the longest life time of SHRSP given 40 mg/kg ONO-3144 were longer than those of normotensive rats and no administration hypertensive rats. The oxidative f ree radicals closely relate to hypertension-induced pathophysiological changes in SHRSP, and ONO-3144 prevents the changes of those disorders, and then brings longevity.