BACKGROUND: Baroreceptor reflex responds to the decrease in blood pressure caused by drug, dehydration, or severe bleeding. Vagal reflex caused by direct pressure on vagus nerve, the traction, or lung inflation develops bradycardia and severe arrhythmia. Lung inflation elicits a vasodepressor reflex, resulting in stimulation of the vagus nerve which causes a decrease in sympathetic outflow. METHODS: 75 patients who had elective surgery were divided into 3 groups according to the age, such as group I: < or =15 years of age, group II: 16-64 years of age, group III: > or = 65 years of age. Lung inflation test has been performed at 20 cmH2O for 20 seconds. Baroreceptor was stimulated by lowering blood pressure with intravenous infusions of nitroglycerin. Baroreceptor sensitivity was assessed by measuring the decrease in blood pressure. Vagal reflex sensitivity was calculated by the subtraction of G2 (baroreceptor reflex sensitivity after introglycerin infusion) from G1 (heart rate response to lung inflation). RESULTS: Baroreceptor reflex induced by hypotension and vagal reflex originated from lung influe-nced the heart rate inversely when lung inflated. Baroreceptor reflex sensitivity was highest in younger patients and lowest in older patients when nitroglycerin infused. Vagal reflex sensitivity was highest in older patients and lowest in younger patients. CONCLUSIONS: Baroreceptor reflex was most sensitive in younger patients, but vagal reflex was moresensitive in older patients.