Mediates the electroneutral exchange of acylcarnitines (O-acyl-(R)-carnitine or L-acylcarnitine) of different acyl chain lengths (ranging from O-acetyl-(R)-carnitine to long-chain O-acyl-(R)-carnitines) with free carnitine ((R)-carnitine or L-carnitine) across the mitochondrial inner membrane, via a ping-pong mechanism. Key player in the mitochondrial oxidation pathway, it translocates the fatty acids in the form of acylcarnitines into the mitochondrial matrix, where the carnitine palmitoyltransferase 2 (CPT-2) activates them to undergo fatty acid beta-oxidation. Catalyzes the unidirectional transport (uniport) of carnitine at lower rates than the antiport (exchange). (R)-carnitine(out) + O-acetyl-(R)-carnitine(in) = (R)-carnitine(in) + O-acetyl-(R)-carnitine(out) (R)-carnitine(out) + O-acyl-(R)-carnitine(in) = (R)-carnitine(in) + O-acyl-(R)-carnitine(out) (R)-carnitine(out) + O-propanoyl-(R)-carnitine(in) = (R)-carnitine(in) + O-propanoyl-(R)-carnitine(out) (R)-carnitine(out) + O-hexadecanoyl-(R)-carnitine(in) = (R)-carnitine(in) + O-hexadecanoyl-(R)-carnitine(out) (R)-carnitine(out) + O-octanoyl-(R)-carnitine(in) = (R)-carnitine(in) + O-octanoyl-(R)-carnitine(out) (R)-carnitine(in) = (R)-carnitine(out) Widely expressed, with highest levels in the liver, intermediate levels in heart, testis and kidney and low levels in brain, including cortex, cerebellum, hippocampus and hypothalamus. Belongs to the mitochondrial carrier (TC 2.A.29) family.