The effects of norepinephrine (NE) on hepatic amino acid exchanges were studied in the isolated perfused rat liver using a recirculating system with a medium containing amino acids at twice physiologic concentrations. Norepinephrine induced a significant decrease (25%) in portal blood flow at a concentration of 2 ng/ml (10(-8) M). The hormone also increased total hepatic amino acid uptake, essentially through a switch from glutamine release to net uptake. There was no modification in free intracellular amino acids, but glycogen was slightly decreased, and glucose production was increased. Taken as a whole, these results suggest that NE modulates hepatic protein balance.