The antihypertensive effectiveness and the clinical and biochemical safety of cicletanine were evaluated in 84 patients (28 women, 56 men) presenting with permanent essential hypertension without severe cardiovascular complications. The hypertension was insufficiently controlled by a beta-blocker, a centrally acting antihypertensive drug or nifedipine. After 3 months of treatment during which cicletanine was added to each of these three classes of drugs, there was a significant fall of systolic arterial pressure (-18 mmHg with beta-blockers, -17 mmHg with central agents and -26 mmHg with nifedipine) and diastolic arterial pressure (-22, -21 and -28 mmHg respectively), resulting in normalization of blood pressure (less than 160/95 mmHg) in 63%, 43% and 50% respectively of patients in each therapeutic group. The fall of blood pressure was accompanied by a significant decrease of functional symptoms (headache, palpitations, dizziness); in the nifedipine group, the addition of cicletanine resulted in complete regression of anginal attacks. The therapeutic combinations were well tolerated; only two patients were excluded from the study for undesirable effects unascribable to cicletanine. Otherwise, the side-effects observed were minor. The biochemical values measured did not significantly vary, and the variations noted were of small amplitude.