1. Resting and stimulated free calcium concentrations have been measured in platelets loaded with the fluorescent probe quin2 from 30 patients with essential hypertension and from 30 age-matched controls. 2. Cytosolic free calcium concentrations were 94.6 +/- 2.7 (mean +/- SEM) in the hypertensive group and 91.7 +/- 2.8 nmol/l in the normotensive group, the difference was not significant. 3. Arginine vasopressin caused a transient increase in platelet free calcium concentration in all subjects. In the presence of extracellular calcium the increase was significantly higher in the control subjects than in the hypertensive patients (P = 0.005). In the absence of extracellular calcium, arginine vasopressin caused much smaller increases, and there was then no difference between the responses of the two groups. 4. Platelet free calcium concentrations were measured again in 13 patients after 8 weeks treatment with either verapamil (n = 6) or atenolol (n = 7). The reductions in systolic pressure after drug treatment were correlated with the changes in cytosolic free calcium concentrations (r = 0.75, P less than 0.01).