We reported recently that peroxisome proliferator-activated receptor beta (PPARbeta) activation promotes a calcineurin-dependent exercise-like remodelling characterised by increased numbers of oxidative fibres and capillaries. As physical exercise also induces myonuclear accretion, we investigated whether PPARbeta activation alters myonuclear density. Transgenic muscle-specific PPARbeta over-expression induced 14% increase of myonuclear density. Pharmacological PPARbeta activation promoted rapid and massive myonuclear accretion (20% increase after 48 h), which is dependent upon calcineurin/nuclear factor of activated T cells signalling pathway. In vivo bromodeoxyuridine labelling and proliferating cell nuclear antigen immunodetection revealed that PPARbeta activation did not promote cell proliferation, suggesting that the PPARbeta-promoted myonuclear accretion involves fusion of pre-existing muscle precursor cells to myofibres rather than cell division. Finally, we showed that in skeletal muscle, ageing led to a down-regulation of PPARbeta accompanied by decrease of both oxidative fibre number and myonuclear density. PPARbeta pharmacological activation counteracts, at least in part, the ageing-driven muscle remodelling.