In heart failure,it has been recognized that sympathetic hyperactivity occurs, which is well counteracted by beta-blockers. In addition, there is frequently parasympathetic deficiency, which in turn contributes to the deterioration of cardiac function. The aim of the study was to evaluate the parasympathetic function by the deep breathing test and to highlight the deficiency of this system in the heart failure patient. We conducted a descriptive cross-sectional study from March to August of 2019. Patients were subdivided into 2 groups:group 1 included 35 patients with chronic heart failure (left ventricular ejection fraction ≤49%) and group 2 (control group) included 32 patients without heart failure(left ventricular ejection fraction ≥50%). The exploration of the parasympathetic system in our patients was based on the deep breathing test. It revealed a vagal deficit in 60% of group 1 patients versus 20% of group 2 patients. This difference was statistically significant between the 2 groups with a p-value of 0.032 (<0.05). Theseresults showed a significant vagal deficiency in heart failure patients and were in line with the literature. Therefore, vagal stimulation in patients with heart failure may be a good therapeutic option to improve symptoms and cardiac functionas some studies have shown.