1. A colony of stroke-prone spontaneously hypertensive rats has been developed by selective breeding.
2. These animals developed severe hypertension early in life, the magnitude of the hypertension being closely related to the incidence of stroke.
3. No evidence was obtained of any humoral factor responsible for strokes.
4. Local factors predisposing to stroke were a scanty arterial supply with characteristic recurrent branching of long and large arteries, together with increased vascular permeability, angio-necrosis, and formation of microaneurysms.
5. Strokes could be prevented by adequate anti-hypertensive therapy from an early age.