The pressor effects of a single infusion of calcium gluconate (1375 mg) were measured in 20 patients, aged 31-63 years, with mild and moderate essential hypertension, being on long-term treatment with the slow calcium channel blocker verapamil. The calcium load induced a significant (P less than 0.001) increase in mean serum ionized calcium (from 1.24 +/- 0.01 to 1.40 +/- 0.2 mmol/l). This did not alter mean blood pressure or mean heart rate, although the individual patients responded differently to the mild hypercalcaemia; a significant fall in blood pressure being observed in a few patients. These results demonstrate the unpredictable effects of an increase in extracellular calcium on vascular smooth muscle cells and suggest that an intravenous bolus of 1375 mg calcium gluconate is not effective in counteracting the hypotensive action of verapamil.