Spontaneously hypertensive rats (SHR) newborn to three months of age exhibit significantly lower serum inorganic phosphate concentrations than do controls from two normotensive strains, Sprague Dawley (CD) and Wistar Kyoto (WKY). During early postnatal development serum inorganic phosphate increases in all strains and plateaus from eight to twelve weeks. This rise in serum phosphate occurs during a period when blood pressure is increasing in normotensive strains as well as in SHR. Hypophosphatemia even in prehypertensive SHR may relate to elevated parathyroid hormone levels in SHR and may explain the salutary effects of high calcium diets in this hypertensive model.