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also provided evidence of the complex expressions of multiple cardiac regions which act simultaneously. It was demonstrated specifically that the body surface location of potential maxima and minima cannot always be equated simply with the cardiac location of responsible events when multiple regions are active simultaneously. The body surface location of maxima and minima are, however, only one feature of body surface potential patterms. The contour of isopotential lines, the potential gradients at various loca-tions, and the time sequence of changing potential values, gradients, and isopotential contours also are descriptors whose utility for regional cardiac examination should be examined. Findings from this study suggest that individual cardiac regions have different effects of body surface potential patterns and support the possibility of regional cardiac examination by electrocardiography. Our findings also demonstrate, however, that simultaneous effects of multiple cardiac regions have complex surface potential manifesta-tions. The use of arithmetic manipulations of maps provided insights concerning the distribution of potentials when more than one dipolar cardiac source was present, and this method is likely to be useful in analysis of maps for clinical diagnosis.8 Separation of effects due to individual cardiac areas, when multiple areas have simultaneous effects, is the major problem for development of regional electrocardio-graphic examination of the heart