1. The amount of urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG), a biomarker of the total systemic oxidative stress in vivo, in stroke-prone spontaneously hypertensive rats (SHRSP) was not different from that in control normotensive Wistar-Kyoto (WKY) rats at 6 weeks of age, but became higher than control values after the development of severe hypertension at 14-17 weeks of age. 2. The amount of urinary 8-OHdG was not significantly different between SHRSP treated with anti-hypertensive agents and those not treated at 14 weeks of age. 3. Stroke-prone spontaneously hypertensive rats were exposed to DNA damage by oxidative stress at the early stage when they developed severe hypertension, but this increase in DNA damage was not the secondary effect of hypertension.