Bisoprolol is a new cardioselective beta-blocker with a long half-life. The efficacy of once daily bisoprolol (10 mg) and atenolol (100 mg) was assessed in 20 patients with stable angina using a placebo controlled double-blind randomized crossover protocol. Efficacy was assessed by computer assisted treadmill exercise testing with monitoring of leads CM5 and CC5, carried out 22-24 h after the last dose. The mean +/- SEM exercise time on placebo was 6.5 +/- 0.4 min increasing to 7.8 +/- 0.5 min on bisoprolol (P less than 0.001) and 8.6 +/- 0.6 mins on atenolol (P less than 0.001). The time to 1 mm ST depression in CM5 and CC5 was also prolonged significantly with both drugs. The mean basal resting heart rate of 84 +/- 4 bpm decreased to 63 +/- 2 bpm on bisoprolol (P less than 0.001) and 64 +/- 3 bpm on atenolol (P less than 0.001), with a significant decrease in the peak exercise heart rate seen with both drugs (P less than 0.001). The peak rate-pressure product was 175 +/- 8 after placebo, 146 +/- 7 (P less than 0.001) with bisoprolol and 149 +/- 5 (P less than 0.001) after atenolol. One patient was withdrawn because he suffered a myocardial infarction. Eighteen patients were prescribed bisoprolol 10 mg once a day for 6 weeks and an exercise test was performed at the end of this period. Bisoprolol retained its efficacy at the end of this period and was well tolerated.(ABSTRACT TRUNCATED AT 250 WORDS)