Calpains have been proposed to be involved in the cytoskeletal remodeling and wasting of skeletal muscle. However, few data are available about the specific involvement of each calpain in the early stages of muscle atrophy. The aim of this study was to determine whether calpains 1 and 2 are autolysed after a short period of muscle disuse and, if so, where in the myofibers the autolysed products are localized. In rat soleus muscle, five days of immobilization increased autolysed calpain 1 in the particulate and not the soluble fraction. Conversely, autolysed calpain 2 was not found in the particulate fraction whereas it was increased in the soluble fraction after immobilization. In the less atrophied plantaris muscle, no difference was noted between the control and immobilized groups whatever the fraction or calpain. Other proteolytic pathways were also investigated. Ubiquitin proteasome was activated in both skeletal muscles and caspase 3 was activated only in the soleus muscle. Taken together, our data suggest that calpains 1 and 2 are involved in atrophy development in slow type muscle exclusively and that they have different regulation and protein targets. Moreover, the activation of proteolytic pathways appears to differ in slow and fast muscles and the proteolytic mechanisms involved in fast type muscle atrophy remain unclear.