Graded maximal treadmill exercise responses were studied before and after beta blockade (atenolol 100 mg once daily for 2 weeks) in 20 male patients with chronic stable angina. Beta-blocking effect consisted of significant reduction of resting heart rate (HR) by 21%, systolic blood pressure (SBP) by 12% and rate pressure product (RPP) by 30%. While the maximum exercise capacity was marginally increased by mean 1.7 min +/- 1.6 SD (P less than 0.001) under the influence of therapy, peak HR, SBP and maximum RPP were significantly lower (P less than 0.001) than in preatenolol exercise tests. Similarly, while the configuration and magnitude of ST segment depression did not differ materially between the pre and post atenolol tests, onset time of ST change was delayed and offset time shortened significantly. These parameters cannot be relied upon to assess the extent and severity of coronary artery disease (CAD) if stress test is carried out while the patient is on a beta-blocking drug. The overall sensitivity of the stress test to detect coronary disease is, however, not likely to be compromised because of negligible influence of beta-blockers upon ST segment depression provided maximally tolerated (not submaximal) exercise is performed. ST/HR slope, an exercise test variable known to correlate well with the extent of CAD, was shown to be uninfluenced by beta-blockade. Its measurement is therefore recommended in interpreting stress tests performed in patients receiving beta-blocker therapy. This, however, requires a meticulously prepared protocol of recording computer averaged QRST complexes and multilead ECG tracings at very frequent intervals throughout the exercise.(ABSTRACT TRUNCATED AT 250 WORDS)