External K+inhibits the maximal rate of outward Na+, K+cotransport in human red cells with no effect on the apparent affinity for internal Na+. The K+concentration giving halfmaximal inhibition (KIK) varied from 16 to 30 mM in 24 normotensive control subjects. Six of the 38 hypertensive patients showed a KIKabove the upper limit of this normal range. Only three hypertensive patients showed a KIKbelow normal range. The internal Na+content giving half-maximal stimulation of outward Na+, K+cotransport (KSNawas measured in the hypertensive patients (a normal range of KSNa= 9 to 16 mmol/liter cells was previously established in 50 normotensive control subjects). Eighteen hypertensive patients showed an abnormally high KSNa, as previously described in hypertensive patients whose Na+, K+cotransport system had a low affinity for internal Na+(Co⊖). Comparison of KSNa, with KIKshowed that all six hypertensive patients with high KIKand all three hypertensive patients with low KIKwere Co ⊖ hypertensive.