Skeletal muscle is highly adaptable, being capable of undergoing changes in its structural and functional properties in response to physiological stimuli. The fast-to-slow muscle fiber-type transition is evoked by increased motor nerve activity. Recently, the calcineurin (CaN) signaling pathway has been implicated in the transcriptional regulation of slow muscle fiber genes. Here we investigated the effect of treatment with a CaN-specific inhibitor, FK506, on skeletal muscle fiber-type transition in functionally overloaded muscles. The overloaded plantaris muscle showed fast-to-slow muscle fiber type transition, i.e., a decrease in myosin heavy chain (MHC) IIb, an increase in MHCIIa+d/x, and new expression of MHCI. In the FK506-administered group, however, overload-induced muscle fiber-type transition was completely prevented. We have demonstrated, therefore, that the CaN signaling pathway is required for fast-to-slow skeletal muscle fiber-type transition. Furthermore, we also confirmed that the protein expression levels of downstream effectors of CaN signaling exhibit a transient increase in the early phase of the overloaded condition.