Although exercise testing is commonly used to determine the efficacy of antianginal drugs, there is little information on the effect of frequent exposure to such testing over periods of long as 6 mo. In or study 10 patients (four men and six women) with stable angina pectoris received placebo for 6 mo. Treadmill testing followed a modified Bruce protocol. All patients exercised to an end point of typical anginal pain and 1 mm or more of ST depression. The first treadmill test for diagnostic purposes was followed by testing every 2 wk for 6 mo. Sublingual nitroglycerin was permitted to abort attacks of angina. Parameters evaluated included heart rate, double product, and duration of exercise. There was no change in the maximal heart rate (mean = 109 at 2 wk and 112 at 6 mo) or double product (mean = 17,002 at 2 wk and 17,249 at 6 mo). On the other hand, duration increased (mean 7.8 min at two wk and 9.9 min at 6 mo). Thus, although treadmill testing showed reproducible measurements of maximal heart rate and double product over 6 mo, exercise duration increased progressively.