We examined whether the quantity of exercise performed influenced the expression of MCT1 and MCT4 in mouse skeletal muscles (plantaris, tibialis anterior, soleus) and heart. Wheel running exercise (1,3 and 6 weeks) was used, which results in marked variations in self selected running activity. Differences in muscle MCT1 and MCT4 among animals, prior to initiating running were not related to the quantity of exercise performed on the first day of wheel running. No changes in MCT4 were observed over the course of the study (P>0.05). After 6 weeks of running were there significant increases in heart (50%, P<0.05) and muscle MCT1 (31-60%, P<0.05), but not after 1 and 3 weeks (P>0.05). As skeletal muscle MCT1, and running distances varied considerably, we examined the relationship between these two parameters. Within the first week of training, MCT1 was negatively correlated with the accumulated running distance (r = -0.70, P<0.05). Upon further analysis, it appears that in the first week, excessive running (>20 km/week) represses MCT1 (-16.1 %, P<0.05), while more modest amounts of running (<20 km/week) increased MCT1 (+37%, P<0.05). After 3 weeks of running a positive relationship was observed between MCT1 and running distance (r = +0.76), although there is a threshold that must be exceeded before an increase over the control animals occurs. Finally, in week 6, when MCT1 was increased in the TA and PL muscles, there were no correlations with the accumulated running distances. These studies have shown that mild exercise training fails to increase MCT4 and that changes in MCT1 are complex, depending not only the accumulated exercise, but also on the stage of training.