BACKGROUND The onset of acute myocardial infarction (MI) is accompanied by a rapid increase in electrical instability and often fatal ventricular arrhythmias. The aim of this study was to assess the continuous arrhythmia profile during the initial 48 hours after coronary artery ligation in the rat in relation to time course, mortality, and infarct size. RESULTS Continuous ECG recording were obtained in 26 conscious, untethered rats for 24 hours before and 48 hours after coronary ligation by use of an implantable telemetry system. All episodes of ventricular tachycardia and fibrillation were counted and their durations summed. Infarct size was measured at 48 hours after MI or after spontaneous death. After ligation, two distinctly active arrhythmogenic periods developed (A1, 0 to 0.5 hours; A2, 1.5 to 9 hours), each followed by a quiescent phase of low ectopy (Q1, 0.5 to 1.5 hours; Q2, 10 to 48 hours). The total mortality rate of 65% was found within the two active periods, with 13 of 15 deaths occurring in A2. Rats with larger infarcts (> or = 50%) and nonsurvivors tended to have increased arrhythmia frequency and duration compared with both animals with smaller MIs (< 50%) and survivors. CONCLUSIONS Two distinct arrhythmogenic periods occur in rats with acute MI that may be caused by different mechanisms and correspond to the bimodal arrhythmia time course seen in dogs and humans after acute MI. Telemetric monitoring of the ECG in the conscious rat after infarction will be useful in assessment of the differential effects of therapeutic interventions on these two arrhythmogenic periods and in the study of potential mechanisms for the spontaneous resolution of ventricular ectopy and risk of sudden death.