INTRODUCTION: Nitroglycerin (NTG) challenge during head-up tilt-table testing is often utilized to determine the aetiology of unexplained vascular syncope. Conflicting results concerning nitrate-induced changes in haemodynamics during tilt-table testing have been reported. The purpose of the present study was to compare the underlying haemodynamic mechanisms in subjects without and with presyncopal symptoms during a nitroglycerin-stimulated tilt-table test. METHODS: Thoracic impedance cardiography (NiccomoTM, medice, Germany) and plethysmographic finger blood pressure were recorded in a supine position and during head-up tilt in 40 patients admitted with a history of vascular syncope or dizziness. 28 subjects showed presyncopal symptoms (11 male/17 female, age 32 짹 15 years) after inhalation of nitroglycerin spray (Isoket) and 12 subjects (8 male/4 female, age 43 짹 18 years) showed no presyncopal symptoms. The drug was administered in the passive head-up tilt followed 5 min later. RESULTS Supine and upright haemodynamics were similar before nitroglycerin administration in the two groups, but the decrease in systemic vascular resistance (SVR) was greater (1,170짹352 vs 1,359짹538 dyn•s•cm-5, P=0.18) in presyncope group. During the nitroglycerin-stimulated tilt test, Finger systolic and diastolic blood pressure decreased significantly more in the presyncope group when compared with the controls (P= 0.005 and P= 0.004, respectively). The decreases in systemic vascular resistance (P= 0.006), while the change in cardiac index was not different between the groups (P= 0.14). The systemic vascular resistance (SVR) was significantly lower in presyncope group (1,036짹472 vs 1,531짹441 dyn•s•cm-5 , P=0.03) during nitroglycerin stimulation. CONCLUSIONS: Presyncopal symptoms during nitrate-stimulated tilt test were explained by decreased systemic vascular resistance, while cardiac output remained unchanged. These findings indicated reduced arterial resistance in nitroglycerin-induced presyncope.