Plasma ionized calcium levels have been shown to be lower than normotensive control levels in hypertensive patients with low plasma renin activity and higher than control levels in hypertensive patients with high renin activity; they did not differ between high and low plasma renin activity groups of normotensive controls. To see if ionized calcium may have different relationships with blood pressure across renin categories in normotensive individuals, plasma ionized calcium was measured on 875 healthy individuals, ages 3 to 83, who had never been diagnosed as having hypertension. Blood pressures were measured in the sitting, standing, and supine positions, along with pressures measured during two stress maneuvers: isometric handgrip and a 50 degrees tilt from a supine position. There was no linear correlation of blood pressure with plasma ionized calcium in the entire sample of youths or adults. However, after dividing the adults into tertiles based on plasma renin activity, there were significant inverse correlations between ionized calcium and systolic and diastolic blood pressure in the low renin group (r = -0.16 to -0.25, P less than or equal to .05), while the systolic blood pressure correlations were significantly positive in the high renin group (r = 0.14 to 0.22, P less than or equal to .05). Adults with normal renin levels did not have any significant correlations of plasma ionized calcium with blood pressure. These confounding effects of renin were greater for systolic than for diastolic blood pressure. These correlations within renin tertiles occurred even though there were no differences in mean blood pressure, plasma ionized calcium, total plasma protein and plasma sodium across renin categories.(ABSTRACT TRUNCATED AT 250 WORDS)