Six hundred and seventy-six healthy subjects between 5 and 77 years of age have been studied by radioelectrocardiography. These subjects were studied before exercise, during a Master's two-step test, and after exercise. Orthostatic ST-T changes occurred in 32 of 200 subjects (16 per cent). The results indicate that the positioning of the bipolar chest leads is crucial in eliminating "abnormal" ST-T changes caused by changes in posture and position. Criteria for orthostatic ST-T changes have been presented. It is concluded that more rigid criteria for "definitely abnormal" ST-T changes during radioelectrocardiography should be developed, so that coronary artery disease will not be overdiagnosed, nor will significant ST-T changes be regarded as "normal variants" occurring with activity.