Disuse of postural slow-twitch muscles, as it occurs in hypogravity, induces a slow-to-fast myofibre type transition. Nothing is known about the effects of weightlessness on the resting membrane chloride conductance (gCl), which controls sarcolemma excitability and influences fibre type transition during development and adult life. Using the current-clamp method, we observed that rat hindlimb unloading (HU) for 1-3 weeks increased gCl in fibres of the slow-twitch soleus (Sol) muscle toward values found in fast muscle. Northern blot analysis suggested that this effect resulted from an increased ClC-1 chloride channel mRNA level. In the meantime, a 4-fold increase in fibres expressing fast isoforms of the myosin heavy chain (MHC) was observed by immunostaining of muscle sections. Also, Sol muscle function evolved toward a fast phenotype during HU, as demonstrated by the positive shift of the threshold potential for contraction. After 3-days HU, Sol muscle immunostaining and RT-PCR experiments revealed no change in MHC protein and mRNA expression, whereas the gCl was already maximally increased, due to a pharmacologically probed, increased activity of ClC-1 channels. Thus the increase in gCl is an early event in Sol muscle experiencing unloading, suggesting that gCl may play a role in muscle adaptation to modified use. Pharmacological modulation of ClC-1 channels may help to prevent disuse-induced muscle impairment.