Chronic low-frequency electrical stimulation of rabbit fast-twitch skeletal muscle induces increased levels of two intermediate filament proteins, desmin and vimentin, during the first 3 weeks of stimulation. These increases occur over the same timecourse as reported shifts in alpha-actinin expression and increased Z-disc width, but precede the fast-to-slow shifts in contractile proteins, which have been described by others. Desmin and vimentin levels increase during the first 2 weeks of stimulation, at which time the increase in desmin appears to plateau while vimentin continues to increase significantly through 3 weeks of stimulation. Absolute amounts of vimentin are lower than desmin at all time points, however increases in desmin and vimentin levels are strongly correlated during the stimulation period, suggesting that the two proteins are coordinately increased during the initial phases of muscle transformation. We suggest that rapid increases in the expression of intermediate filament proteins, which coincide with alterations in Z-disc structure, may indicate a fortification of the force-bearing ultrastructure of the muscle fibre in response to the increased activity that is induced by stimulation. The presence of vimentin and elevated levels of desmin expression suggest that mature skeletal muscle reverts toward a developmental program of intermediate filament protein expression during fast-to-slow transformation.