The emergence of ventricular tachycardia (VT), its temporal progress and response to high-frequency stimulation were studied in 10 dogs subjected to two-step occlusion of the left descending coronary artery (LDA). Within two hours after the LDA ligation under atrioventricular block all animals developed an atrioventricular nodal rhythm of 37 +/- 9 pulses per minute that could be suppressed by high-frequency ventricular stimulation, i.e. the so-called overdrive suppression (OS) phenomenon was occurring. Three or four hours after LDA ligation, atrioventricular block brought out VT in 8 dogs. The onset of VT was always abrupt, its episodes being short-lived at first and growing progressively longer with time. Once VT was established, its rate increased gradually to reach the peak that exceed the base line VT rate by 21 +/- 9% 2 or 3 hours later. As the VT rate increased, the OS phenomenon grew less pronounced and disappeared altogether as the VT peak was reached. The results suggest that the abrupt emergence of VT 3 or 4 hours after the onset of myocardial infarction can be a result of ectopic pacemaker activity of partially depolarized fibres that in some cases may be due to the effects of a trigger mechanism.