Precordial ST-segment mapping was serially applied in the Coronary Care Unit for the study of the effect of oxygen inhalation on the ischemic injury in 17 patients with acute anterior transmural myocardial infarction. A 49-lead ECG system was used. The sum of all ST elevations (sigmaST) recorded was taken as an index of magnitude of ischemic injury and the number of recording sites showing ST elevation (NST) was taken as an index of extent of ischemic damage. Stability of the precordial maps was observed over a period of one hour while the patients were on ambient air. Oxygen inhalation for a mean of 66 min resutled in a fourfold increase of PaO2 and a mean of 16% reduction of both sigmaST and NST. When the patients were returned to ambient air breathing, a mean of 13% increase of sigmaST and 19% of NST from the levels recorded during oxygen inhalation were observed. Levels of sigmaST and NST on ambient air following discontinuation of oxygen inhalation were not significantly different from the corresponding values from maps recorded before onset of oxygen breathing. Blood pressure and heart rate remained unchanged throughout the study. Clinical status of the patients was unchanged during the study period save for two patients who showed changes in intensity of their chest pain.