To examine the relationship between net hepatic glucose uptake (NHGU) and the insulin level and to determine the effects of portal glucose delivery on that relationship, NHGU was evaluated at three different insulin levels in seven 42-h-fasted, conscious dogs during peripheral glucose delivery and during a combination of peripheral and portal glucose delivery. During peripheral glucose delivery, at arterial blood glucose levels of approximately 175 mg/dl and insulin levels reaching the liver of 51 +/- 2, 92 +/- 6, and 191 +/- 6 microU/ml, respectively, NHGUs were 0.55 +/- 0.30, 1.52 +/- 0.44, and 3.04 +/- 0.79 mg/kg per min, respectively. At hepatic glucose loads comparable to those achieved during peripheral glucose delivery and inflowing insulin levels of 50 +/- 4, 96 +/- 5, and 170 +/- 8 microU per ml, respectively, NHGUs were 1.96 +/- 0.48, 3.67 +/- 0.68, and 5.52 +/- 0.92 mg/kg per min when a portion of the glucose load was delivered directly into the portal vein. The results of these studies thus indicate that net hepatic glucose uptake is dependent on both the plasma insulin level and the route of glucose delivery and that under physiological conditions the "portal" signal is at least as important as insulin in the determination of net hepatic glucose uptake.