A novel fast-twitch motor unit type, called the IIX-myosin heavy chain (MHC) motor unit, identified by the glycogen depletion technique together with a series of monoclonal antibodies (mAbs) specific for MHCs, has been isolated recently in the rat tibialis anterior muscle. In young animals, this unit has physiological, biochemical and morphometrical properties which separate it from the IIA- and IIB-MHC motor units. In old age, on the other hand, the IIX-MHC units display physiological, biochemical and morphometrical properties resembling the IIB-MHC motor units. Based on these results it was proposed that a transition from IIB to IIX motor units occurs during ageing. In an attempt to clarify this point, the MHC composition was identified by 6% SDS-PAGE and immunoblotting analysis, using specific mAbs antibodies, in the same fast-twitch tibialis anterior muscles in young (3–6 months, n = 9) and old (20–24 months, n = 16) rats from which the single motor units had been identified previously. The IIX-MHC comigrates together with the HA-MHC band in 6% SDS-PAGE and only two MHC bands are observed in the rat tibialis anterior muscle, i.e. the IIA-IIX-and IIB-MHC bands. A significant increase (P < 0.001) in the average relative amount of the HA-IIX —MHC was observed in the old (45 ± 17%) as compared to the young (23 ± 4%) animals, accompanied by a corresponding decrease in IIB-MHC content. It was demonstrated in immunoblotting analysis that only trace amounts of IIA-MHC were detectable in the IIA-IIX-MHC band in both young and old TA muscles, indicating a substantial increase in the IIX-MHC content in old age. Thus, the present results together with previous observations at the motor unit level strongly support an age-related motor unit transition from type IIB- to IIX-MHC.