The effects of vagal nerve stimulation in the neck and in the lower thorax were studied on L-leucine absorption in/the cat jejunum. Four series of in vivo experiments were performed in 26 cats using single pass perfusion technique with samples collected every 10 min and examined for their phenol red and radioactive L-leucine concentrations. The first two series acted as controls for the time it took to reach steady-state absorption and for the effects on L-leucine absorption of mechanical handling of the vagus nerve. In the third series, vagal stimulation in the neck (with a 20-second train of shocks at 7 Hz repeated every 10 min for either 16 or 4 stimulus sessions) resulted in a 3-fold increase in L-leucine absorption which was maintained throughout the stimulation period (up to 160 min). L-Leucine absorption started increasing within 10 min and peaked around 30-40 min after the initiation of stimulation and returned to control levels within 40 min after cessation of stimulation. In the fourth series, similar increases and returns to control L-leucine absorption levels were noted following vagal stimulation in the lower thorax (with a 20-min train of shocks at 7 Hz given for 3 sessions). The findings suggest a definite role of the vagus nerve in the control of leucine transport across the small intestine.