1. The effect of long-term treatment with prindolol on blood pressure, total body potassium (Kt), exchangeable sodium (Nae) and plasma renin activity was investigated in twelve patients with essential hypertension.

2. Systolic and diastolic pressures were significantly reduced from 164/112 to 127/90 mmHg under basal conditions.

3. Before treatment Na. in patients with essential hypertension was significantly higher than in normotensive individuals. After an average of 16 weeks on prindolol Nae in patients with essential hypertension was significantly decreased, despite an average increase in body weight of 2 kg in the patients.

4. In contrast to the decrease in Nae, Kt was found to be significantly increased after long-term treatment with prindolol. Kt values of patients before and after prindolol, however, did not differ significantly from the corresponding sex- and age-dependent normal values.

5. Plasma renin activity was slightly diminished under basal and orthostatic conditions; the stimulatory effect of orthostasis was not abolished but reduced by prindolol.

6. It is suggested that the changes in sodium balance contribute to the anti-hypertensive effect of prindolol in patients with essential hypertension.

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