The effect of torso inhomogeneities on the electrocardiographic body surface potentials in general, and the waveform of the QRS complex in the standard leads in particular, were studied by using an inhomogeneous, numerical model of the torso including the (lower) conductivity of the lungs and the (higher) conductivity of the ventricular cavities. The relevant geometry was derived from actual data as measured from NMR scans of the subjects studied. The simulated potentials were compared to the body surface potentials recorded in the same individuals. In view of the observed high quality of the correspondence in the fully inhomogeneous case, the quantitative results of the study of the effects of the involved inhomogeneities attain a high level of credibility. To facilitate this analysis a new tool is introduced: the contribution maps.