1. The intracellular concentrations of Na+, K+ and Ca2+ were measured in the erythrocytes of spontaneously hypertensive rats and normotensive Wistar rats.
2. The intracellular Na+ concentration in hypertensive rats was slightly elevated at 3.16 ± 0.25 compared with 2.85 ± 0.35 mmol/l (P ≈ 0.05) and intracellular Na+ activity was markedly increased in hypertensive rats.
3. Intracellular Ca2+ activity was 7519 ± 28 990 nmol/l of free water in hypertensive rats compared with 123 ± 98 in controls (P < 0.01).
4. The cytoplasm of hypertensive animals did not buffer Ca2+ as effectively as that of normal animals.
5. It is concluded that a decreased binding capacity of intracellular macromolecules for Na+ and Ca2+ may explain the disturbances of intracellular electrolyte composition in spontaneously hypertensive rats.