Platelet survival was studied by using "Cr-labeled platelets in stroke-prone spontaneously hypertensive rats (SHRSP), stroke-resistant SHR (SHRSR) and normotensive control rats of the Wistar-Kyoto (WK) strain. Relatively young animals of the same age prior to the development of cerebrovascular lesions (cerebral infarction and/or hemorrhage) were used.Platelet half-life time in SHRSP was slightly but significantly shorter than in any other groups of rats, irrespective of the type of platelet donors. Mean platelet consumption was also significantly increased in SHRSP only. Platelets of SHRSP injected into SHRSR showed normal survival. These data support the concept that the shortened platelet survival in SHRSP is brought about by some extracorpuscular abnormalities. Although the vascular changes in SHRSP could be the most likely explanation for the shortened platelet survival, its mechanism remains to be solved. This investigation suggests that studies of the platelet survival in hypertension may be useful in predicting the development of stroke before its clinical recognition.