The acute effect of glyceryl trinitrate (GTN) is mainly the reduction of systolic blood pressure (SBP) rather than diastolic blood pressure (DBP). To examine the mechanisms for decrease in SBP by GTN, the authors measured arterial compliance (AC), cardiac output (CO), stroke volume (SV), pulmonary capillary wedge pressure, right ventricular end-diastolic pressure, right atrial pressure, and left ventricular end-diastolic pressure before and after administration of GTN. Fourteen patients who underwent cardiac catheterization were included in this study. The change of each variable was estimated by the differences between values just before and at three minutes after GTN. After administration of GTN, SBP was significantly decreased from 158 +/- 21 to 138 +/- 12 mmHg (P < 0.01). AC was significantly increased, and pulmonary capillary wedge pressure, CO, and SV were significantly decreased. The change in SBP was inversely correlated with the change in AC and positively correlated with the change in CO or SV. The authors conclude that the mechanism for decrease in SBP by GTN is associated with both an increase in AC and a decrease in venous return.