High blood pressure in acute stroke is common and appears to be associated with a poor outcome. Lowering blood pressure might therefore improve outcome, provided that cerebral perfusion is not compromised. We assessed the effects of glyceryl trinitrate (GTN) on cerebral and systemic hemodynamic measures in acute stroke. Ninety patients with acute ischemic or hemorrhagic stroke were randomized within 72 hours of ictus to transdermal GTN given daily for 10 days (either 5 mg, 5 mg for 4 days then 10 mg, or 10 mg) or control. Twenty-four hour blood pressure monitoring, middle cerebral artery blood velocity, cardiac output, augmentation index, and plasma nitric oxide levels were each measured at baseline and then on days 1, 4, 5, and 10. The primary outcome was blood pressure on day 1. We found that GTN lowered mean peripheral arterial blood pressure on day 1 by 5.3% to 6.7% in a dose dependent manner as compared with control (mean, SD): control, 108.8 (15.1) mmHg; 5 mg, 102.5 (13.9) mmHg; 5/10 mg, 103.4 (14.9) mmHg; 10 mg, 101.5 (12.6) mmHg; (P = .005). Increasing the dose from 5 to 10 mg on day 5 resulted in an overall reduction in blood pressure of 11.4% as compared with leaving the dose at 5 mg (P = .006). GTN reduced peripheral pulse pressure, central aortic blood pressure, pulse pressure, and augmentation index on day 1. Middle cerebral artery blood velocity and pulsatility index in the affected hemisphere, cardiac output, systemic peripheral resistance, and plasma nitric oxide levels were not altered by GTN. Treatment with GTN was associated with headache: control 0 (0%), GTN 9 (15%) (P = .027); no negative effect on end-of-treatment death or deterioration, or 3 month death or dependency was discernable. GTN reduced peripheral blood pressure in a dose-dependent fashion in patients with acute stroke at day 1 and also reduced central blood pressure and augmentation index. In contrast, GTN did not alter middle cerebral artery blood velocity or pulsatility index in the affected hemispheres, suggesting that cerebral blood flow did not change. A trial assessing the effect of lowering blood pressure with GTN on safety and functional outcome in patients with acute stroke is now warranted.