We propose to evaluate the effect of bezafibrate on metabolism during exercise in adult patients affected with CPT II or VLCAD deficiencies. This study will be an 11-month, randomized double blind, placebo-controlled crossover trial. The trial will be conducted in two centers: Institut de Myologie, Pitié-Salpêtrière Hospital in France and Neuromuscular Research Unit, Rigshospitalet, University of Copenhagen, in Denmark. Since exercise test on cycle ergometer after stable isotopes infusion is probably the most reliable way to assess in vivo fat oxidation, this technique will be utilized as the one of two major outcomes for this study, and the main criteria for assessing the potential effect of this drug will be the fat oxidation rate during a moderate workload on cycle ergometer.