Objectıve: Propagation of depolarisation and repolarisation in myocardium results from an interplay of membrane potential, transmembrane current, and intercellular current. This process can be represented mathematically with a reaction-diffusion (RD) equation. Solving RD equations for a whole heart requires a supercomputer. Therefore, earlier models used predefined action potential (AP) shapes and fixed propagation velocities. We discuss why RD models are important when T waves are studied. Methods: We simulated propagating AP with an RD model of the human heart, which included heterogeneity of membrane properties. Computed activation times served as input to a model that used predefined AP, and to a “hybrid model” that computed AP only during repolarisation. The hybrid model was tested with different spatial resolutions. Electrocardiograms (ECGs) were computed with all three models. Results: Computed QRS complexes were practically identical in all models. T waves in the fixed-AP model had 20 to 40% larger amplitudes in leads V1–V3. The hybrid model produced the same T waves as the RD model at 0.25-mm resolution, but underestimated T-wave amplitude at lower resolutions. Conclusion: Fixed AP waveforms in a forward ECG model lead to exaggerated T waves. Hybrid models require the same high spatial resolution as RD models.