Male stroke prone spontaneously hypertensive rats (SHRSP) were fed 4% NaCl diet containing either 0.75% normal potassium or 2.11% high-potassium, starting at 6 weeks of age. After 8 months on these diets, 40 out of 58 SHRSP on 0.75% potassium had died (69% mortality) versus two dead out of 95 on 2.11% potassium (2% mortality), a 97% reduction in mortality, P less than 0.000 01. After 20 weeks on the diet, the daytime and night-time blood pressures (BPs) of each rat were measured intra-arterially under light ether. Using these accurate BPs, we selected two groups precisely matched for BP. One matched SHRSP group (BP 182) ate the 0.75% potassium diet and 30 out of 47 rats died (64% mortality). The other matched SHRSP group (BP 182) ate the 2.11% potassium diet and two out of 35 died (6% mortality) a 91% reduction of mortality, P less than 0.0001. Seemingly, the striking reduction in mortality rate with the 2.11% high-potassium diet does not depend on a lowering of BP. High-potassium diets do not change muscle, aorta or body sodium or potassium. Dry weight of mesenteric arterioles was reduced 29% on the 2.11% potassium diet versus the 0.75% potassium diet [5.43 mg versus 7.66 mg](P less than 0.0001) indicating a greatly reduced hypertensive hypertrophy, even though BP was equal in the two groups being compared. Aortic wall wet weight was reduced 25.5% in 36 rats on a 2.11% potassium diet versus 26 rats on a 0.75% potassium diet (36.7 mg versus 49.2 mg) P less than 0.001, even though BP was equal in the two groups being compared. In nine surviving SHRSP rats on 0.75% potassium, 13 of 36 brain hemisphere slides (four slides per rat) showed infarcts (36%). In 11 surviving SHRSP rats on 2.11% potassium, one of 44 brain slides showed infarcts (2%, a 95% reduction) P less than 0.0001. In other SHRSP rats on a 0.75% diet for 8 weeks, 18 of 25 rats (72%) had spots of brain haemorrhages whereas only two of 36 rats (5.5%) on 2.11% potassium had similar haemorrhages-a 92% reduction (P less than 0.000 01). High-potassium diets allow cerebral arteries to carry very high BPs without sustaining damage to the artery wall, thereby drastically reducing brain haemorrhages and infarcts and lowering the death rate. Moreover, hypertension does not invariably lead to artery hypertrophy, since a high-potassium diet can prevent most of it.