A computer model of a 3-dimensional rectangular block of myocardial tissue (3969 cells) has been used to investigate the influence on excitation and repolarisation sequences and on the modelled electrocardiographic T wave of (a) electrotonic interaction, (b) intrinsic distribution of refractoriness, and (c) the speed of repolarisation of action potentials. The model allowed electrotonic interactions to be investigated separately during the depolarisation and repolarisation phases. Scales of 14 values of the strength of electrotonic interaction during the depolarisation phase, 14 values of the strength of electrotonic interaction during the repolarisation phase, 3 shapes of action potential, and 5 distributions of tissue refractoriness were selected and all 2940 combinations were examined. In each experiment, the tissue model was artificially excited and the resulting excitation and repolarisation sequences were simulated. The results of the study suggested that electrotonic interactions between excited cells can cause non-uniform speed of propagation which, by means of the phase shifts of action potentials, contributes to the inversion of the repolarisation sequence and to the physiologic orientation of T waves. Experiments with this model did not support the hypothesis that simple electrotonic smoothing of the differences in repolarisation phases due to the excitation phase shift of action potentials reverses the repolarisation sequence and explains T wave polarity.