BACKGROUND Sodium and potassium ions are involved in the regulation of blood pressure and the genesis of hypertension. OBJECTIVE To assess internal potassium balance, as a measure of sodium pump activity, in subjects with essential hypertension and diabetic patients. METHODS Eleven hypertensive subjects, 5 non-insulin-dependent diabetics and 16 age matched controls were studied. An acute oral load of 0.8 mEq/Kg body weight of KCl was administered and blood samples were drawn every 30 min thereafter, until 120 min, to measure plasma K+ levels. Urinary K+ excretion during this period was also measured. In eight hypertensive patients, the test was repeated after two week of supplementation with 60 mEq/day of KCl. The maximal increase in plasma potassium levels and the time required to achieve the maximum concentration was recorded. RESULTS All patients had normal serum creatinine levels. Mean fasting blood glucose of diabetic patients was 133 +/- 15.1 mg/dl. No difference between patients and controls in maximal increase plasma potassium increase, was observed. In hypertensive patients the lapse to achieve the maximal potassium concentration was longer than in controls. After the period of potassium supplementation in hypertensive patients, there was a significant increase in basal plasma K+ levels and the temporal pattern of plasma potassium increase was similar to that of controls. Between 63 and 68% of retained K+ load was translocated to the intracellular space at 120 min in all study groups. CONCLUSIONS Internal potassium balance is not significantly altered in subjects with essential hypertension or in non-insulin-dependent diabetics.