The training intensity may affect the monocarboxylate transporters MCT1 and MCT4 in skeletal muscle. Therefore, 20 elite cross-country skiers (11 men and nine women) trained hard for 5 months at either moderate (MIG, 60-70% of VO2max) or high intensity (HIG, 80-90%). The lactate threshold, several performance parameters, and the blood lactate concentration (cLa) after exhausting treadmill running were also determined. Muscle biopsies taken from the vastus lateralis muscle before and after the training period were analysed for the two MCTs and for muscle fibre types and six enzymes. The concentration of MCT1 did not change for HIG (P=0.3) but fell for MIG (-12 +/- 3%, P=0.01); the training response differed between the two groups (P=0.05). The concentration of MCT4 did not change during the training period (P > 0.10). The concentration of the two MCTs did not differ between the two sexes (P=0.9). The running speed at the lactate threshold rose for HIG (+3.2 +/- 0.9%, P=0.003), while no change was seen for MIG (P=0.54); the training response differed between the two groups (P=0.04). The cLa after long-lasting exhausting treadmill running correlated with the concentration of MCT1 (rs=0.69, P=0.002), but not with that of MCT4 (rs=0.2, P=0.2). There were no other significant correlations between the concentrations of the two MCTs and the performance parameters, muscle fibre types, or enzymes (r < or = 0.36, P > 0.10). Thus, the training response differed between MIG and HIG both in terms of performance and of the effect on MCT1. Training at high intensity may be more effective for cross-country skiers. Finally, MCT1 may be important for releasing lactate to the blood during long-lasting exercise.