Duplex sonography was used to assess the effects on hepatic blood flow after administering 0.6 mg nitroglycerin (NTG) sublingually to ten healthy volunteers. The study was a randomized, placebo-controlled, cross-over study in which subjects were studied on three separate occasions. Each visit involved administering either placebo or NTG followed by estimation of blood flow through a particular branch of the hepatic artery, portal vein, and hepatic vein every minute for 15 minutes after NTG and placebo administration. Two hours later, subjects were crossed over to the other treatment and the same vessel branch was again examined for 15 minutes. Total blood flow increased 7% in the portal vein and 27% in the hepatic vein during NTG treatment, but did not change significantly in the hepatic artery. Vascular resistance was increased in the hepatic artery and decreased in the portal and hepatic veins after NTG. Qualitatively, flow changed dramatically in the hepatic vein after NTG with the disappearance of normal retrograde flow. The results indicate that nitroglycerin effects hepatic blood flow through the portal and hepatic veins with a decrease in vascular resistance in the portal and hepatic veins and an increase in resistance in the hepatic artery.