Praecordial surface maps of the electrocardiogram were recorded before and after exercise using 16 electrodes covering the left hemithorax. The ST segment and R and S wave changes were measured in the praecordial maps from 20 individuals with no detectable cardiovascular disease. These showed no significant alteration in ST segments of R/S. In contrast in 40 patients with angiocardiographically proven coronary artery disease it was possible clearly to outline the distribution, severity, and time course of praecordial areas of ST segment depression (36 patients) and ST segment elevation (10 patients). In addition these praecordial areas of ST segment changes were accompanied by a regional and significant fall in the R/S. The praecordial electrocardiogram with exercise complements the anatomical information obtained from the coronary arteriogram by clearly outlining electrocardiographic projections of regional myocardial ischaemia or cell death.