We have examined the effects of 7-day testosterone administration on MCT1 and 4 mRNAs and proteins in seven metabolically heterogeneous rat hindlimb muscles and in the heart. In addition, we also examined the effects of testosterone treatment on plasmalemmal MCT1 and MCT4, and lactate transport into giant sarcolemmal vesicles prepared from red and white hindlimb muscles and the heart. Except for the plantaris muscle, testosterone did not alter MCT1 or 4 mRNA. Testosterone increased MCT1 (20-77%, p < 0.05) and MCT4 protein (29-110%, p < 0.05) in 5 out of 7 muscles examined. In contrast, in the heart MCT1 protein was not increased (p > 0.05), and MCT 4 mRNA and protein were not detected. There was no correlation between the testosterone-induced increments in MCT1 and MCT4 proteins. Muscle fiber composition was not associated with testosterone-induced increments in MCT1 protein. In contrast, there was a strong positive relationship between the testosterone-induced increments in MCT4 protein and the fast-twitch fiber composition of rat muscles. Lactate transport into giant sarcolemmal vesicles was increased in red (23%, p < 0.05) and white muscles (21%, p < 0.05), and in the heart (58%, p < 0.05) of testosterone-treated animals (p < 0.05). However, plasmalemmal MCT1 protein (red +40%, p < 0.05; white +39%, p < 0.05) and plasmalemmal MCT4 protein (red +25%, p < 0.05; white +48%, p < 0.05) were increased only in skeletal muscle. In the heart plasmalemmal MCT1 protein was reduced (-20%, p < 0.05). In conclusion, these studies have shown that testosterone induces an increase in both MCT1 and MCT4 proteins and their plasmalemmal content in skeletal muscle. However, the testosterone-induced effect was tissue-specific, since MCT1 protein expression was not altered in the heart. In the heart, the testosterone-induced increase in lactate transport cannot be explained by changes in plasmalemmal MCT1, but in skeletal muscle the increase in the rate of lactate transport was associated with increases in plasmalemmal MCT1 and MCT4.