The effects of exercise on skeletal muscle are mediated by a coupling between muscle electrical activity and gene expression. Several activity correlates such as Ca²⁺, hypoxia and metabolites like free fatty acids (FFAs) might initiate signaling pathways regulating fibre-type-specific genes. FFAs can be sensed by lipid-dependent transcription factors of the peroxisome proliferator-activated receptor (PPAR) family. Our objective was to investigate the role of the predominant muscle isoform, PPAR, in the pathways connecting muscle activity to the regulation of fibre phenotype. In this study, PPAR mRNA and protein were measured in the slow soleus and the fast extensor digitorum longus (EDL) muscles of adult rats, and mRNA was quantified after electrical stimulation with a "fast" and "slow" pattern, respectively. Furthermore, a constitutively active fusion protein, VP16-PPAR, was overexpressed in the EDL by in vivo transfection, and the muscles were analyzed histochemically 5 or 14 days after transfection. Endogenous PPAR mRNA was found to be three-fold higher in the slow soleus compared to the fast EDL and on histological sections, the most oxidative fibres displayed the highest levels of PPAR protein. After electrical stimulation with "mismatch" patterns, the mRNA level in the soleus was reduced to less than half within 24 h, while increased 3-fold in the EDL. Overexpression of VP16-PPAR by somatic gene transfer resulted in smaller fibre cross sectional area, increased succinate dehydrogenase activity and a higher proportion of I/IIa and IIa fibre types. Our data suggest that PPAR can mediate activity effects acutely in pre- existing adult muscle fibres, and thus is an important link in the excitation-transcription coupling.