Calcium has been implicated in smooth muscle contraction, arterial resistance, and the pathophysiology of essential hypertension. Using the intracellularly trapped fluorescent dye quin2 , the free calcium concentration in platelets was found to be elevated in patients with borderline (n = 8, p less than 0.01) and established essential hypertension (n = 23, p less than 0.001) when compared with normotensive subjects (n = 30). There was a close correlation between intracellular free calcium and systolic blood pressure (n = 61, r = 0.882, p less than 0.001) as well as diastolic blood pressure (n = 61, r = 0.950, p less than 0.001). The slopes of the regression lines did not differ between the groups.