The transport of histidine and glutamine via system N in cultured hepatocytes was found to be subject to hormonal control. This long-term regulation showed the following characteristics.1. The transport capacity for histidine and glutamine (system N) increased slowly in response to the combination of dexamethasone and insulin to about 4-fold that of controls after 18-30 h. A similar time course was found for the stimulation of system N (2.5-fold) by dexamethasone and glucagon. In contrast the uptake of a-aminoisobutyric acid (system A) was rapidly stimulated 3-fold by dexamethasone and insulin and 5-fold by dexamethasone and glucagon within 3 -6 h but decreased towards control rates after 24 h of cultivation in minimal essential medium.2. Dexamethasone, insulin and glucagon each stimulated glutamine uptake about 2-fold in cultures maintained in W/AB 77 medium, while the combination of dexamethasone with either glucagon or insulin resulted in a 3 -4-fold increase.3. Dexamethasone was most effective at about 0.1 pM. Higher concentrations were less efficient. Insulin reached its optimal effect at concentrations above 1 pM.4. Kinetic analysis revealed that the increased capacity of glutamine transport in response to hormones was due to an increase in V,,,, while K,,, was essentially unchanged.5. The hormone-induced stimulation of system N was prevented by cycloheximide. 6. The induced uptake of glutamine was inhibited by excess amounts of asparagine and histidine but not of These results clearly differentiate the hormonal regulation of system N from that of system A.