The purpose of this study was to investigate the changes of heart rate (HR) and sympathetic tone occurring after myocardial infarction. Male rats (200-250 g) were submitted to ligation of the anterior branches of the left coronary artery to produce infarction (INF, N = 26) or to sham surgery (SO, N = 24). Groups of animals (N = 6-8) were studied 1,3,7 and 15 days later. A group not submitted to surgical procedures was used as control (C,N = 6). Blood pressure and HR were measured in conscious unrestrained animals after cannulation of the femoral artery. Basal HR and mean arterial pressure (MAP) of the C group were 336 +/- 8 beats/min (bpm) and 110 +/- 3 mmHg, respectively. Similar values were observed in SO subgroups. One day after INF, HR increased to 417 +/- 9 bpm and MAP decreased to 97 +/- 2 mmHg. While MAP was unchanged for the remainder of the study, HR progressively decreased and normal HR values (360 +/- 18 bpm) were observed two weeks after INF. The sympathetic tone, which was evaluated from the reduction of HR after propranolol (2 mg/kg, iv), was increased during the tachycardic phase after INF. HR normalization paralleled the progressive reduction of sympathetic tone. The changes in HR after coronary artery ligation in rats seem to reflect the degree of sympathetic efferent activity during infarct healing.