This article shows the use of the equivalent surface source model in its application to the genesis of the surface ECG in which the sources are of the double layer type. This model has previously been shown to yield an accurate description of body surface potentials during the QRS interval. Its application to the genesis of the T wave is now worked out in greater detail. In this source model, the full spatio-temporal character of the cardiac electric generator is expressed by means of an equivalent double layer situated at the boundary of ventricular tissue. The timing of local depolarization and of repolarization of the cells near this boundary is used to simulate the surface ECG. The background of this source model is discussed, as well as its validity. A matrix formulation is presented of the forward problem of computing the resulting body surface potentials. Based on this forward formulation, an inverse computation of the timing of the repolarization process was performed. Simulated T waves, based on this timing, showed a close correspondence with measured body surface potentials. By taking the inversely computed timing of repolarization as representing the true situation, the model can be used to study in which way perturbations in the timing of local repolarization effect the shape of T waves. An analysis based on this model indicates that the amplitude of the T wave is proportional to the dispersion of the repolarization times at the ventricular surface. The model is also shown to be capable of simulating the ECG changes during local ischemia.