1. We have investigated the function of the Na+-K+ pump in the erythrocytes of patients with various types of hypertension by measuring the activity of ouabain-sensitive Na+-K+-dependent ATPase and ouabain-insensitive ATPase in the erythrocyte ghosts of these patients.

2. Mean Na+-K+-ATPase activity was increased threefold in six patients with Cushing's syndrome and hypertension (0.985 ± sd 0.288 μmol of phosphate h−1 mg−1) compared with that in 49 control subjects (0.334 ± 0.147 μmol of phosphate h−1 mg−1) (P<0.0025), but not in four patients with primary aldosteronism (0.278 ± 0.108 μmol of phosphate h−1 mg−1).

3. Na+-K+-ATPase activity slightly exceeded the control range in 57 patients with primary hypertension (0.410 ± 0.245 μmol of phosphate h−1 mg−1; P<0.025) and in 12 patients with renal hypertension (0.475 ± 0.250 μmol of phosphate h−1 mg−1) (P<0.025).

4. There was no difference in ouabain-insensitive ATPase between the controls and the different types of hypertension.

5. The data support the hypothesis of an activation of the Na+ pump in patients with glucocorticoid excess. This helps to explain previous observations of a redistribution of sodium and water to the extracellular compartment in experimental glucocorticoid-induced hypertension.

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