The efflux of 22Na from erythrocytes of spontaneously hypertensive rats (SHR) and the ones of Wistar and Spra glue-Dawley aged 8 weeks was studied. The efflux rate of the isotope from erythrocytes with ouabaine blocked active transfer of ions was much higher in the SHR in the prehypertensive and early hypertensive stages than in Wistar and Sprague-Dawley rats. These data suggest that, by comparisons with the Wistar and Sprague-Dawley the SHR have an elevated passive permeability of the erythrocytes membrane to sodium. The authors consider this fact as a possible indication of the presence in spontaneous hypertension of a more extensive defect of the membranous permeability which can prove to be a major cause responsible for activation of servomechanisms that keep up the arterial pressure at a high level.