In order to compare the medium term antihypertensive effectiveness and tolerability of atenolol with those of bopindolol (LT 31-200), a new non-selective beta-blocker with slight PAA (partial agonist activity), a randomized double-blind study was performed. Thirty-one outpatients with mild-to-moderate essential hypertension (WHO stage I-II) were enrolled and after a placebo run-in randomly allocated to bopindolol (1 to 4 mg/day) or atenolol (50 to 200 mg/day). The dose was titrated according to the individual pressor responses, and thereafter it was kept constant until the end of the treatment (12 weeks). Both drugs induced statistically significant decreases in SBP, DBP and HR, both in resting conditions and during an ergometric test. Accordingly, most patients achieved the main goal of the therapy, i.e., supDBP less than or equal to 90 mmHg, 11/15 (74%) with bopindolol versus 8/13 (62%) with atenolol. There were no significant differences between the effects of the two compounds. Resting airway resistance (expressed as Peak Expiratory Flow) was not influenced by the treatments. The antihypertensive efficacy was still evident after 12 months in 8 patients who were evaluated for non-comparative long-term effectiveness of bopindolol monotherapy, and improvements in plasma lipid profiles were also observed. Side effects were relatively mild and transient, with two patients in the atenolol group discontinuing therapy (one for inefficacy and one because of undesirable reactions) and one dropped out in the bopindolol group (late evidence of not fulfilling inclusion criteria).(ABSTRACT TRUNCATED AT 250 WORDS)