1. The effects of aldosterone in vitro on the Na+,K+-dependent ATPase activity of isolated human erythrocyte membranes and on rubidium (86Rb) uptake and [3H]ouabain binding of intact erythrocytes were studied.
2. ATPase activity was nearly doubled (0.061 ± 0.006 to 0.110 ± 0.01 μmol of Pl h−1 mg−1 of protein) by the addition of a physiological concentration of aldosterone (2.7 × 10-10 mol/l). Higher concentrations had no greater effect.
3. Aldosterone had no significant effect on 86Rb uptake or [3H]ouabain binding.
4. Erythrocytes contain aldosterone at concentrations similar to that in plasma. The effect of aldosterone on ATPase is probably maximal.