An in vitro luminal perfusion technique was used to monitor the transepithelial movement of [3H]L-carnitine into the cauda epididymal lumen of rats. Movement of the volume marker [14C]inulin into the lumen was limited while the transport of [3H]L-carnitine was saturable and was dependent on respiratory energy. The transport of L-carnitine (50 microM) was significantly inhibited by 500 microM D-carnitine, palmitoyl L-carnitine and deoxycarnitine but not by trimethylammonium iodide, trimethyl phosphate, acetyl-L-carnitine, choline, betaine, gamma-aminobutyric acid or DL-carnitinamide. The Kt (116 microM) and Vmax (0.1 nmol/30 min/cm) for the carriers were calculated from the present data and as the Kt is about twice the normal plasma concentration of carnitine, the rate of transport by the pump is presumed to be limited by the plasma concentration.