To estimate quantitatively the infarct size of right ventricular infarctions (RVI), I studied the pre- and post-infarction body surface maps of 10 dogs with transmural RVI and 5 dogs with non-transmural RVI. The maximum decrease in the potential at all lead points was 0.8mV through the ventricular depolarization after a sham operation in 4 dog. Thus, a 1mV postoperative decrease was considered significant. In transmural RVI, the minimum and negative potential zone appeared early in the ventricular depolarization on the right anterior chest surface. The negative potential zone continued to the middle stage of depolarization the right chest surface, and afterward occupied a larger part of anterior surface. The maximum and positive potential zone shifted leftward in the early and middle stages of QRS. In transmural RVI, ΣQ16 (the sum total of the voltages which postoperatively decreased 16msec after the onset of QRS) was much greater than the ΣQ16 of non-transmural RVI and sham operation. Especially, reductions of ΣQ16 greater than 15mV were found in transmural RVI. Q waves were found on right precordial leads (V(2R) through V(5R)) in 5 cases of trans-mural RVI. The total number of leads with decreased potential over 1mV (Σn) and ΣQ16 from maps correlated well with the histologically determined infarct size of RVI. These data suggest that Σn and ΣQ16 would be good clinical indices for diagnosing quantitatively the infarct size of RVI. Maps obtained after intravenous administration of isoproterenol enabled the discrimination between non-transmural RVI and sham operation.