1. Because disturbances of calcium metabolism have been described in hypertension, measurements of plasma and serum concentrations of ionized calcium, total calcium, magnesium and renin were made in 38 patients with essential hypertension and age- and sex-matched control subjects. Urinary excretion of calcium, magnesium and sodium was also determined. 2. The mean serum concentration of ionized calcium was 1.23 +/- 0.04 (SD) mmol/l in the hypertensive group and 1.21 +/- 0.03 mmol/l in controls, and results were similar after correction for pH. There was a weak positive correlation between serum ionized calcium (pH 7.4) and systolic pressure (r = 0.26, P less than 0.02), but no correlation with plasma renin concentration. 3. Although the difference between serum total calcium concentration in the hypertensive (2.29 +/- 0.09 mmol/l) and control (2.26 +/- 0.07 mmol/l) subjects was not significant, there was a significant correlation between total calcium and systolic pressure (r = 0.23, P less than 0.05) which was maintained after correction for other variables. 4. There were no differences in plasma concentrations of parathyroid hormone or 1,25-dihydroxycholecalciferol between hypertensive and control subjects. 5. The hypertensive group showed higher urinary excretion of calcium (5.9 +/- 3.0 mmol/24 h) than controls (4.6 +/- 1.7 mmol/24 h), but the difference was not maintained after correction for sodium excretion. 6. Serum concentrations of magnesium were similar in the two groups, but urinary excretion of magnesium was significantly lower in hypertensive (3.7 +/- 1.3 mmol/24 h) than control (4.5 +/- 1.6 mmol/24 h) subjects and there was an inverse correlation between magnesium excretion and blood pressure (r = 0.3-0.35, P less than 0.01).