The aim of this study was to evaluate together the main hemorheologic parameters and one of the transmembrane ion transport systems in erythrocytes of subjects with normal and elevated blood pressure. Three sex-, age-, and weight-matched groups consisting of 15 normotensive subjects (NT) with no parental hypertension, 15 patients with essential hypertension (EH) at stage 1-II WHO, and eight patients with secondary hypertension (Sec.H), respectively, were studied. Red blood cell Na+-Li+ countertransport (CTT), blood viscosity (eta B) at shear rates of 230 X S-1, 115 X S-1, and 46 S-1 and plasma viscosity (eta P) at shear rate of 46 X S-1 were measured. Plasma proteins and fibrinogen were also evaluated. CTT was higher in EH than in NT (P less than 0.01), while no significant difference was found between NT and Sec. H patients. eta B at 115 X S-1 and 46 X S-1 was higher in EH, but not in Sec. H, than in NT patients (P less than 0.05). No difference in eta P, plasma proteins and fibrinogen levels was observed between EH and NT. Elevated eta B and/or CTT may indicate a structural alteration in the erythrocyte membrane of some essential hypertensive patients. This is consistent with the hypothesis that a widespread membrane disorder is involved in the pathogenesis of primary hypertension.