Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors exerting several functions in development and metabolism. The physiological functions of PPARdelta remain elusive. By using a CRE-Lox recombination approach, we generated an animal model for muscle-specific PPARdelta overexpression to investigate the role of PPARdelta in this tissue. Muscle-specific PPARdelta overexpression results in a profound change in fiber composition due to hyperplasia and/or shift to more oxidative fiber and, as a consequence, leads to the increase of both enzymatic activities and genes implicated in oxidative metabolism. These changes in muscle are accompanied by a reduction of body fat mass, mainly due to a large reduction of adipose cell size. Furthermore, we demonstrate that endurance exercise promotes an accumulation of PPARdelta protein in muscle of wild-type animals. Collectively, these results suggest that PPARdelta plays an important role in muscle development and adaptive response to environmental changes, such as training exercise. They strongly support the idea that activation of PPARdelta could be beneficial in prevention of metabolic disorders, such as obesity or type 2 diabetes.