In vivo, insulin increases net hepatic glucose uptake efficiently only in the presence of a portal-arterial glucose gradient. In isolated perfused rat livers supplied with a glucose gradient (portal 10 mM/arterial 5 mM) insulin-induced glucose uptake was blocked by atropine; in livers not supplied with the gradient (portal = arterial 5 mM) insulin-dependent glucose uptake was elicited by acetylcholine. Apparently, the gradient was sensed and transformed into a metabolic signal by intrahepatic nerves, releasing acetylcholine to muscarinic receptors.