1. Blood pressure was measured after treatment with a high K+, a low Na+ and a combined high K+/low Na+ diet in young spontaneously hypertensive rats (SHR).
2. A high K+ diet reduced blood pressure by approximately 10 mmHg during the development of hypertension. This decrease was accompanied by a significant increase in water intake and urine volume and a significant decrease in plasma renin activity (PRA).
3. A low Na+ diet also decreased blood pressure significantly, but, in contrast to the high K+ diet, water intake and urine volume significantly decreased and PRA increased.
4. When both diets were given together, the antihypertensive effects of both were eliminated. Thus while an increase in dietary K+ and a decrease in dietary Na+ are both effective antihypertensive regimens in SHR, the mechanism of action of each appears to be different and may be antagonistic in these animals.