Platelet free Ca2+concentration has been found to be elevated in essential hypertension and to correlate with blood pressure level. Free cytoplasmic calcium concentration is determined by calcium influx, pooling, and efflux. The present study found a Ca2+-ATPase in platelet membranes that has a high afflnity for Ca2+(Km∼-1 μM), is inhibited by low concentrations of orthovanadate (K1— 1 μM), and can be stimulated by calmodulin (Km∼ — 5 nM). The absolute increase in calmodulin-stimulated Ca2+-ATPase activity was not different between normotensive and hypertensive subjects; however, the degree of stimulation of Ca2+-ATPase activity at saturating calmodulin concentrations apparently was diminished in calmodulindeficient membranes from subjects with established essenltial hypertension (40%) as compared to that in normotensive subjects of similar age (135%;p< 0.001). Aflinities for calmodulin and Ca2+were comparable between the two groups, while the capacity for Ca2+-ATPase activity (basal and calmadulin-stimulated) was markedly greater (1.5- to 1.8-fold) in both native and calmodulin-deficient membranes from hypertensive subjects. On the other hand, the defective calcium efflux pump activity, as Bssessed by a decreased degree of calmodulin stimulation, may have contributed to elevated cytoplasmic calcium concentrations and the associated enhanced hormone sensitivity in platelets from essential hypertensive subjects. This may represent an adaptive negative feedback control mechanism to protect the cell against Ca2+overload.