1. In 25 patients with untreated essential hypertension and 25 healthy controls, erythrocyte intracellular Cl− concentration and activity as well as Na+ activity were measured.
2. Intracellular Cl− concentration in essential hypertensive patients was 70.6 ± 11.3 as compared with 84.4 ± 9.5 mmol/l in the controls (P<0.001).
3. Intracellular Cl− activity was 77.5 ± 13.0 mmol/l of cell water in hypertensive patients, the control value being 100.8 ± 11.0 mmol/l of cell water (P<0.001).
4. In the hypertensive group intracellular Na+ activity was 14.3 ± 4.1 as compared with 7.1 ± 1.8 mmol/l of cell water in the normotensive group (P<0.01).
5. From these results it is suggested that in essential hypertension not only disturbances of cation metabolism, but also of anion metabolism, occur. Possibly the Cl− changes reflect a decreased Cl− inward transport due to an altered Na-K co-transport.