The relation between heart rate and ischemic ST-segment depression was studied in 70 patients with documented obstructive coronary artery disease (CAD) and reproducible effort angina. Symptom-limited treadmill exercise testing was performed before and after a 2-week placebo period and 24-hour FM ambulatory electrocardiographic monitoring at the end of the placebo period. The means (+/- standard deviation) of the basal and placebo values for exercise time, heart rate and maximal ST-segment depression were: 6.4 +/- 2.6 minutes vs 6.9 +/- 2.8 minutes (difference not significant [NS]), 125 +/- 17 beats/min vs 125 +/- 19 beats/min (NS) and 2.3 +/- 0.8 mm vs 2.1 +/- 0.8 (NS), respectively. Ambulatory monitoring revealed 205 episodes of significant ST-segment depression (J + 80 ms; 49 episodes with more than 1 mm, 83 with more than 2 mm, 39 with more than 3 mm and 34 with more than 4 mm). Of all episodes of ST-segment depression, 130 (64%) were asymptomatic. The episodes lasted for 3 to 110 minutes. The maximal 24-hour ambulatory heart rate and ST-segment depression during ischemic episodes were expressed as a percentage of those seen during exercise-induced ischemia. When all ambulatory ischemic episodes (both symptomatic and asymptomatic) were compared with exercise-induced ischemic changes in the individual patient, there was little difference in heart rate (91 +/- 15% vs 90 +/- 18%, NS) but there was a greater magnitude of ST-segment depression (122 +/- 57% vs 104 +/- 52%, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)