1. The relationship of reciprocal change on the electrocardiogram, at the time of acute myocardial infarction, to exercise-induced ST segment depression and coronary anatomy was studied in 125 post-infarct patients. 2. Eighty-three patients had reciprocal changes, 90 had exercise-induced ST depression and 72 had both of these findings. 3. Patients with reciprocal changes had larger myocardial infarctions, as assessed by peak enzyme release and ejection fraction, than patients without this finding. 4. Multi-vessel disease was significantly more common among patients with reciprocal changes and those with exercise-induced ST depression compared with patients without these findings. 5. The exercise test was more sensitive and had a higher predictive accuracy than reciprocal change when electrocardiographic changes were compared with findings at coronary angiography. 6. With both tests the antero-lateral leads were significantly more sensitive, but less specific, than the inferior leads in classifying patients. 7. Thus while both tests yielded information with regard to coronary anatomy in post-infarct patients, the exercise test was a better predictor of coronary anatomy than reciprocal change. 8. Therefore, reliance should not be placed on the presence or absence of reciprocal change alone when assessing patients for further investigation after myocardial infarction.