The effectiveness and safety of cicletanine hydrochloride, the first representative of the furopyridine family, were evaluated in a 90-day double-blind study involving 120 patients with moderate essential hypertension poorly controlled after at least one month of treatment with a beta-blocker. After a 30-day pre-inclusion period during which a placebo capsule was given together with a stable dose of the beta-blocker, the patients were randomised to one of three therapeutic groups: group 1 (placebo, n = 40), group 2 (cicletanine 50 mg/day, n = 41), group 3 (cicletanine 100 mg/day, n = 39). All three groups were matched in every respect. Eight patients in group I were excluded (5 for ineffectiveness, 2 for unexpected effect, 1 for intercurrent disease) as was 1 patient in group 3 for unexpected effect. On entering the active phase of treatment, supine blood pressures were 171.3 +/- 13.6/103.9 +/- 6.1 mmHg in group 1, 173.5 +/- 12.7/103.6 +/- 5.2 mmHg in group 2 and 171.8 +/- 15.4/104.5 +/- 5.9 mmHg in group 3. A significant (p less than 0.0001) treatment effect on SBP was found in groups 2 and 3 and on DBP in all three groups. The improvement observed in both SBP and DBP was similar in groups 2 and 3 and highly significant when compared with group 1 (p 0.001). At the end of the trial, 5% of group 1 patients, 51.2% of group 2 patients and 74.4% of group 3 patients had normal blood pressure values. The drug was well tolerated clinically and biochemically.(ABSTRACT TRUNCATED AT 250 WORDS)