Preeclampsia is characterized by a generalized vasoconstriction and increased vascular sensitivity to angiotensin II. Intracellular free calcium, implicated in vascular smooth muscle contraction, has been found to be elevated in platelets of other hypertensive disorders. We therefore measured intracellular free calcium concentrations by using the fluorescent probe quin-2 in platelets of six patients with preeclampsia and compared them to measurements in ten normotensive pregnant women and ten age-matched nonpregnant women. Intracellular free calcium was also determined in the preeclamptic women after delivery. We found that intracellular free calcium was slightly elevated in normal pregnancy (102 ± 13 nmol/L v 87 ± 17 nmol/L) but was markedly increased in pre-eclampsia (138 ± 13 nmol/L, P < .05). This increase disappeared six weeks after delivery (84 + 10nmol/L, P < .01). To investigate whether the increased intracellular free calcium was related to angiotensin II, the platelets were exposed to thrombin and angiotensin II in vitro. Exposure to thrombin and angiotensin II caused a dose-dependent increase in intracellular free calcium. The intracellular response to thrombin was not significantly different in the three groups. However, stimulation with angiotensin II revealed an increased response in intracellular free calcium in preeclampsia (P < .05) that disappeared after delivery. Our findings show a sustained increase in platelet intracellular free calcium in preeclampsia and suggest a functional alteration of the angiotensin II receptor in this disease. Am J Hypertens 1989; 2:238-243