Outward bumetanide-sensitive Na+-K+ cotransport was studied in the erythrocytes of 51 subjects, 24 normotensive subjects and 27 hypertensive patients, matched for sex and age. Three kinetic parameters of this cation transport system were considered: velocity of efflux at saturating internal sodium (Nai) concentrations (Vmax.), apparent affinity for sodium (K 50%) and index of co-operativity among Nai sites (Hill's n). We correlated these values with clinical and laboratory data determined routinely in hypertension. There were no significant differences between normotensive and hypertensive subjects for the values considered and we did not find any significant correlations between co-transport and clinical data.