The carnitine carrier from rat liver mitochondria was purified by chromatography on hydroxyapatite and celite and reconstituted in egg yolk phospholipid vesicles by adsorbing the detergent on polystyrene beads. In the reconstituted system, in addition to the carnitine/carnitine exchange, the purified protein catalyzed a uni-directional transport (uniport) of carnitine measured as uptake into unloaded proteoliposomes as well as efflux from prelabelled proteoliposomes. In both cases the reaction followed a first-order kinetics with a rate constant of 0.023-0.026 min-1. Besides carnitine, also acylcarnitines were transported in the uniport mode. N-Ethylmaleimide inhibited the uni-directional transport of carnitine completely. The uniport of carnitine is not influenced by the delta pH and the electric gradient across the membrane. The activation energy for uniport was 115 kJ/mol and the half-saturation constant on the external side of the proteoliposomes was 0.53 mM. The maximal rate of the uniport at 25 degrees C was 0.2 mumol/min per mg protein, i.e. about 10 times lower than that of the reconstituted carnitine transport in exchange mode.