Disturbed phosphate (PO4) metabolism has been documented in spontaneously hypertensive rats but poorly studied in humans. Twenty-seven drug-free hypertensive subjects were studied on both a 10- and 100-mmol sodium diet. The response of mean arterial pressure to sodium repletion was directly correlated to the response of plasma renin activity (r = 0.540, p = 0.004) and inversely related to the percent response of serum PO4 concentrations. In the sodium-replete state serum PO4 concentration correlated inversely with plasma vitamin D concentration (r = 0.419, p = 0.026), consistent with PO4 acting as a determinate of vitamin D production. The response of serum calcium and plasma vitamin D concentrations to sodium repletion were correlated (r = 0.392, p = 0.043), consistent with serum calcium levels being a dependent variable, but the responses of serum PO4 and vitamin D concentrations were not. This study suggests that PO4 metabolism may be a determinate of blood pressure response to sodium repletion.