The relationship between either heart rate or diastolic time and ST segment depression has been evaluated during supine and upright exercise in 16 coronary artery disease patients. Diastolic perfusion time and ST segment depression were related by a linear regression, which was independent of exercise posture. The entity of ST segment depression was greater during supine than in upright exercise for the same heart rate. The assessment of the relationship between heart rate and diastolic perfusion time during two exercises showed that at the same heart rate, diastolic perfusion time was shorter in supine posture. In conclusion, the greater entity of ST segment depression induced by supine rather than upright exercise might be explained by the effect of supine posture on diastolic perfusion time.