The action of insulin (0.1 U/ml) on the metabolism of human intestinal smooth muscle was studied in vitro. The experiments were performed on the muscle layer of human jejunum obtained from patients undergoing intestinal shunt operations because of obesity. Insulin significantly increased glucose uptake, glycogen content, the membrane transport of α-amino-isobutyric acid (AIB), the incorporation of leucine into protein and tended to increase the membrane transport of the nonutilizable model monosaccharide 3-0-methylglucose. The effects of insulin were moderate and appeared after incubation times of 120 to 180 min.