The effects of insulin on amino acid transport were studied in freshly prepared suspensions of isolated hepatocytes from adult rats. Insulin stimulated the active transport of alpha-aminoisobutyric acid by increasing the influx. The onset of the insulin effect was delayed by thirty to sixty min. Insulin increased by Vmax of transport by about 60% without affecting the Km. Cycloheximide and actinomycin D inhibited hormonal action by 60 to 80%. Only the "A" system of transport was affected by insulin. Half-maximal stimulation of transport was observed with insulin at 2 to 3nmol/l, a concentration which also occupies about 50% of insulin-specific binding sites at steady state. Insulin did not antagonize the stimulatory effect of glucagon on amino acid transport.