Acute myocardial infarction (MI) is known to elicit autonomic responses, depending on its location: inferior-wall MI (Inf-MI) promotes vagal activation, and anterior-wall MI (Ant-MI) promotes sympathetic activation. Reperfusion is thus expected to induce an opposite shift in autonomic balance. We studied the patterns of heart rate variability (HRV), which reflect the autonomic response to reperfusion by applying a continuous wavelet transform (CWT), ECG traces, 3 h long, were obtained from 9 Ant-MI and 8 Inf-MI patients during acute MI. Marked alterations in HRV occured in all reperfusion events. A shift in balance towards sympathetic predominance was found in all Inf-MI patients, and in 3 Ant-MI patients, while a shift towards vagal predominance was found in 6 Ant-MI patients (p<0.001). The CWT thus enables to study cardiac control during non-steady-state conditions, as exhibited during thrombolysis.