Whether subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria contain monocarboxylate transporters (MCTs) is controversial. We have examined the presence of MCTI, 2, and 4 in highly purified SS and IMF mitochondria. These mitochondria were not contaminated with plasma membrane, sarcoplasmic reticulum or endosomal compartments, as the marker proteins for these sub-cellular compartments (Na+-K+-ATPase, Ca2+-ATPase, and the transferrin receptor) were not present in SS or IMF mitochondria. MCTI, MCT2, and MCT4 were all present at the plasma membrane. However, MCTI and MCT4 were associated with SS mitochondria. In contrast, the IMF mitochondria were completely devoid of MCT1 and MCT4. However, MCT2 was associated with both SS and IMF mitochondria. These observations suggest that SS and IMF mitochondria have different capacities for metabolizing monocarboxylates. Thus, the controversy as to whether mitochondria, can take up and oxidize lactate will need to take account of the different distribution of MCTs between SS and IMF mitochondria. (C) 2004 Elsevier Inc. All rights reserved.