The authors report their experience of the evaluation of autonomic nervous system control by measuring the sensitivity of the baroreceptor reflex (SBR) in the animal and in humans after myocardial infarction. The SBR is expressed as the ratio between the variations in heart rate and systolic blood pressure recorded after an injection of phenylephrine. In the dog, with an experimental myocardial infarction and submitted to an exercise stress test + ischemia (by occlusion of a second coronary vessel), a fall in the SBR is predictive of ventricular fibrillation: the SBR was 9.1 +/- 6 ms/mmHg in "sensitive" animals who fibrillated, compared with 17.7 +/- 6.5 in "resistant" animals. In addition, if dogs with experimental infarction are submitted to daily physical training, the SBR increases from 5.4 +/- 1.2 to 16.3 +/- 5 ms/mmHg and VF does not occur during exercise stress testing + ischemia. Finally, the SBR before infarction is also predictive of the risk of VF during exercise stress test + ischemia and of mortality in the acute phase of myocardial infarction. In man, the SBR decreases after infarction and recovers at the third month. There is no correlation between the SBR and LV ejection fraction. In a study of 78 patients, 2-year mortality increased from 3 to 40% when the SBR was less than 3 ms/mmHg and from 10 to 50 p. 100 if only patients with LV ejection fractions of less than 50% were considered. The evaluation of autonomic neural reflexes by measuring the SBR is easy to perform at the bedside and provides informations about post-infarction prognosis independent of and complementary to the state of pump function.