The high incidence of arrhythmias in compensated cardiac hypertrophy is related to two independently regulated components-fibrosis and the adaptational phenotypic changes in membrane proteins linked to cardiac hypertrophy, and fibrosis. During the regression of hypertensive cardiopathy in middle-aged spontaneously hypertensive rats, the roles of cardiac hypertrophy and fibrosis can be analysed separately, revealing that both correlate independently with arrhythmias. In an experimental model of myocardial infarction it is possible to prevent arrhythmias with propranolol at the same time as cardiac hypertrophy, despite ventricular fibrosis. Fibrosis would appear to create arrhythmias both by anatomical uncoupling and by a re-entry mechanism generated by the zig-zag propagation of the transverse waveform. Triggered activity and automaticity depend on the membrane phenotype of the cardiocyte. They also play an important role, which is aggravated by myocardial heterogeneity.