SUMMARY Plasma phosphate values are significantly lower in spontaneously hypertensive rats (SHR) than in normotensive Wistar-Kyoto rats (WKY). In this study, we increased plasma phosphate in SHR by a high dietary phosphate intake and followed the effects on blood pressure. Fifteen male WKY and 15 male SHR were housed from 4 weeks of age up to 26 weeks. At 4 weeks of age all SHR manifested a hypophosphatemia compared with age-matched WKY (F= 62, p<0.0003). At 5 weeks of age, the rats were divided into three diet groups: a control group, a group receiving 1.41% (wt/vol) KCI in drinking water, and a group receiving 2% (wt/vol) K 2 HPO 4 KH 2 PO 4 in drinking water. In the control (F=16.2, p<0.02) and KCI groups, (F=36.3, p<0.03), hypophosphatemia persisted throughout the study. The phosphate-supplemented diet normalized plasma phosphate level in SHR but did not change plasma phosphate level in WKY. As a consequence, no difference in plasma phosphate level between WKY and SHR was present in the group receiving additional phosphate from that time on (F= 1.2, p>0.41). The phosphate-supplemented diet significantly decreased systolic blood pressure in both strains. In phosphate-supplemented SHR, a significant decline in systolic blood pressure was observed from 20 weeks of age on (at 20 weeks of age: 222 ± 3 mm Hg for control SHR vs 198 ± 5 nun Hg for phosphate-supplemented SHR; p < 0.0003). In phosphate-supplemented WKY, a slight but significant decrease hi systolic blood pressure was observed from 24 weeks of age on (at 25 weeks of age: 133 ± 1 mm Hg for control WKY vs 125 ± 1 mm Hg for phosphate-supplemented WKY; p<0.0004). The KCI diet had no significant effect on systolic blood pressure in either strain. Plasma electrolyte (sodium, potassium, calcium, and magnesium) levels were not influenced by the two diets throughout the study. We conclude that a high dietary phosphate intake, with concomitant normalization of hypophosphatemia, results in a decrease in blood pressure in SHR. The phosphate-induced hypotensive effect was not apparent until 15 weeks after initiation of the diet. The mechanism behind this effect remains to be elucidated. (Hypertension 9: 96-102, 1987 Epidemiological and clinical data suggest that phosphate metabolism is altered in human hypertension as well. Plasma phosphate has been shown to be lower in Received November 15, 1985; accepted August 8, 1986. patients with essential hypertension. 4 In hyperparathyroidism associated with hypertension, serum phosphate was significantly lower than in hyperparathyroidism in normotensive subjects. 3 An association between lower intakes of phosphorus and a greater risk of being hypertensive has been observed. 6 Moreover, in normotensive subjects plasma phosphate levels were inversely related to blood pressure, 7 suggesting that a decrease in plasma phosphate could be associated with an increase in blood pressure. In contrast, Lau et al. 8 recently suggested that phosphate deficiency mediates the antihypertensive effect of a high calcium diet in SHR. Thei...