Age-related changes in membrane fluidity of erythrocytes in essential hypertension. In the present study, age- and calcium-related changes in the membrane fluidity of erythrocytes were examined in patients with essential hypertension by use of electron spin resonance method (ESR). The erythrocytes were obtained from patients with essential hypertension. We examined the ESR spectra for a fatty acid spin label agent (5-nitroxy stearate) incorporated into the erythrocyte membranes. The values of outer hyperfine splitting and order parameter (S) were significantly higher in subjects with essential hypertension than in the normotensive subjects. This finding indicates that the membrane fluidity of erythrocytes was lower in essential hypertension. Calcium-loading of erythrocytes with the Ca-ionophore A23187 decreased the membrane fluidity (S value was increased) more strongly in essential hypertension than in the normotensive subjects. Furthermore, this Ca-induced change in membrane fluidity was significantly correlated with age in essential hypertension. These results demonstrate that the membrane fluidity of erythrocytes is markedly decreased by calcium, especially in essential hypertension in older patients. This suggests an increased calcium-sensitivity of cell membranes in the aged hypertensive patient. 