The aim of this work was to study the nitrergic innervation in the liver of the cat using immunocytochemical procedures. At the hepatic hilus, a rich plexus of neuronal nitric oxide synthase immunoreactive (nNOS-IR) nerve fibers and ganglia was detected around the interlobular branch of the bile duct. nNOS-IR nerve fibers were observed running with the components of the intralobular portal triads located close to the hepatic hilus, as well as with a few vessels and ducts of the deeper parenchyma. These latter fibers, beside others located in Glisson's capsule, occasionally showed short ramifications entering the parenchyma itself. The present results suggest that, in the cat liver, nNOS is involved in the autonomic control of hepatic blood flow, with a limited role in the regulation of hepatobiliary excretory activity and hepatocellular metabolic function.