Chronic hypertension accompanying early stage kidney disease is characterized by increased vascular resistance, but the underlying processes responsible for the enhanced vascular tone are unclear. We studied free calcium levels in blood platelets with the fluorescent dye quin-2. Platelets have many features in common with vascular smooth muscle cells. The cytosolic calcium concentration in platelets was elevated in 27 renal hypertensive patients, who were compared with 12 normotensive subjects (P less than 0.001). There was a close correlation between the free calcium level and mean blood pressure (r = 0.88, P less than 0.001). Short-term antihypertensive treatment with a calcium entry blocker or a diuretic resulted in a significant reduction in cytosolic calcium (P less than 0.05), and this correlated with the fall in blood pressure (r = 0.95, P less than 0.001). These data suggest an integrative contributory role of calcium in the pathophysiology of hypertension accompanying early stage kidney disease.