The effects of a 2 litre isotonic saline infusion on erythrocyte Na,K pump activity and urinary sodium excretion (UNaV) were evaluated in 20 patients with essential hypertension and 15 normotensive subjects. The effect of preincubation of normal erythrocytes in plasma from hypertensive patients on erythrocyte Na,K pump was also studied. Before saline infusion no significant differences were found between normotensive and hypertensive subjects in the mean values of intracellular sodium and potassium concentration, ouabain-sensitive Na efflux in erythrocytes and urinary sodium excretion. Erythrocyte Na,K pump activity decreased significantly (p less than 0.01) after saline infusion in both groups of subjects. The reduction was significantly lower in hypertensives than in normotensive. delta UNaV was significantly higher in hypertensive patients than in normotensive subjects (25 +/- 4 vs 14 +/- 2 mmol/h; p = 0.04). Only in normal subjects a significant correlation was found between the difference in Na,K pump activity pre and post saline infusion and delta UNaV (r = 0.52; p less than 0.05). Plasma from hypertensive patients obtained before saline infusion significantly (p less than 0.01) inhibited Na,K pump of erythrocytes from normal subjects; plasma taken after saline infusion produced a marked increase of this inhibition, significantly (p less than 0.01) higher than serum taken before the infusion.