The relationships between blood pressure and altered erythrocyte Ca2+-Mg2+-ATPase activity and membrane fluidity were assessed in membranes prepared from Wistar-Kyoto (WKY) normotensive and spontaneously hypertensive (SHR) rats from two different sources (Charles River [n=15] and Harlan OLAC [n=11, WKY; n=13, SHR) and a second filial (F2) generation derived from a WKY and SHR (Charles River) cross [n=100]. Further investigations using essential hypertensive and normotensive subjects assessed membrane fluidity and fatty acid and cholesterol content.;Ca2+-Mg2+-ATPase activity was measured as ATP-dependent 45Ca2+ uptake into inside-out vesicles and membrane fluidity assessed by measurement of polarisation anisotropy of membrane-incorporated fluorescent probes and an excimer forming technique which monitors lateral fluidity. In the essential hypertension analysis [n=18, normotensive; n=29, hypertensive] fluidity was measured using fluorescence polarisation anisotropy, the excimer forming technique and electron spin resonance spectroscopy (ESR).;Conflicting reports were obtained for Ca2+-Mg2+-ATPase activity and lateral fluidity in membranes of rats from the two different sources and contrary to previous studies, no relationship between adult systolic blood pressure and erythrocyte Ca2+-Mg2+-ATPase activity or membrane fluidity was indicated in the F2 rat population. ESR reported a reduction in fluidity in hypertensive patients (P<0.05) which correlated with systolic blood pressure (r=0.6, P=0.05). No difference in membrane fluidity was found using fluorescence polarisation anisotropy or the excimer forming technique and no alterations in membrane fatty acid composition were reported. Cholesterol levels were elevated in patients with a family of hypertension (P<0.05).;These results suggest that Ca2+-Mg2+-ATPase activity and membrane microviscosity are casually unrelated to hypertension in the SHR model.