Leucine uptake in brush-border membrane vesicles purified from rat jejunum is sodium-dependent, sensitive to the membrane electrical potential difference and enhanced by the intravesicular presence of potassium. This last effect is not mediated by the genesis of an electrical potential difference, since potassium activation and electrical potential effects are additive. Sodium-dependent leucine Vmax (1568 +/- 91 pmol/mg per 3 s, is higher in young rats than in adult and old animals. The diffusion component of leucine transport decreases with increasing age. Preloading the vesicles with 100 mM KCl increases leucine Vmax 200% in young animals, 100% in adult and 44% in old animals. The potassium activation is a saturation function of the cation concentration. Leucine uptake in brush border membrane from old animals is less sensitive to the electrical potential difference than in membranes from adult and young animals.