The authors give an account of recent findings on the modulating action of the autonomous nervous system on electrophysiological properties of the heart and on mechanisms by which the vegetative nervous system participates in the development of disorders of the cardiac rhythm. In the development of arrhythmias, in particular the importance of the sympathetic nerve is emphasized. The relationship of stimulation of the sympathetic nerve and the development of arrhythmias may be conditioned: by its great activity, uneven distribution of sympathetic innervation and abnormal reaction of the damaged tissue to stimulation of the sympathetic nerve. Mechanisms by which stimulation of the sympathetic nerve may participate in the development of arrhythmias can be summarized as follows: the sympathetic nerve promotes the automation of natural and ectopic centres, shortens the refractory period of the conduction system, of the atrial and ventricular myocardium, increases the vulnerability of the atria and ventricles, reduces the fibrillation threshold; by induction of subsequent depolarizations it may induce (triggered activity); in the depolarized heart muscle it supports the development of slow forms of impulses and thus helps the development of reentry tachycardias; activation of the sympathetic nerve may cause hypokalaemia which is also an arrhythmogenic factor.