Male spontaneously hypertensive stroke-prone (SHRsp) rats were fed 4% NaCl diets containing either 0.75% (normal) K or 2.11% (high) K, starting at 6 weeks of age. After 8 months on these diets, 40 out of 58 SHRsp rats on the 0.75% K diet had died (69% mortality) versus 2 dead out of 95 on the 2.11% K diet (2% mortality), a 97% reduction in mortality, p less than 0.00001. After 20 weeks on the diets, the daytime and nighttime blood pressures of each rat were measured intraarterially under light ether anesthesia. Using these accurate blood pressures, we selected two groups precisely matched for blood pressure. One matched SHRsp group (BP 182) ate the 0.75% K diet and 30 out of 47 rats died (64% mortality). The other matched SHRsp group (BP 182) ate the 2.11% K diet and 2 out of 35 died (6% mortality), a 91% reduction of mortality, p less than 0.0001. Seemingly, this striking reduction in mortality rate with the 2.11% high K diet does not depend on a lowering of blood pressure. High K diets do not change body Na or K. Dry weight of mesenteric arterioles was reduced 29% on the 2.11% K diet versus the 0.75% K diet (5.43 vs. 7.66 mg) (p less than 0.0001), indicating a greatly reduced hypertensive hypertrophy. In nine surviving SHRsp rats on the 0.75% K diet, 13 of 36 brain hemisphere slides (4 slides per rat) showed infarcts (36%). In 11 surviving SHRsp rats on the 2.11% K diet, 1 of 44 brain slides showed infarcts (2%), a 94.5% reduction, p less than 0.0001.(ABSTRACT TRUNCATED AT 250 WORDS)