1. Young (3-6 months) and old (20-24 months) male Wistar rat soleus muscles were examined for myosin isoform composition, fibre type, contractility and sarcoplasmic reticulum (SR) Ca2+ release properties either in control rats or in rats treated with thyroid hormone (3,5,3'-triiodothyronine, T3) for 4 weeks. 2. T3 treatment had a strong impact on myosin heavy chain (MyHC) and light chain (MyLC) isoform composition in both young and old rats. That is, all single fibres co-expressed type I and IIA (type I/IIA fibres) or type I, IIA and IIX MyHCs (type I/IIAX fibres) after treatment. Slow and fast MyLC isoforms, i.e. MyLC1s, MyLC1f, MyLC2s, MyLC2f and MyLC3, co-existed in each of the type I/IIA and I/IIAX fibres in variable proportions. 3. In old rats the maximum velocity of unloaded shortening (V0) was related to MyHC isoform composition: V0 for type I fibres was less than that for type I/IIA fibres which was less than that for type I/IIAX fibres. In young rats, on the other hand, V0 did not differ between pure type I fibres from controls and those co-expressing type I and type II MyHC isoforms from T3-treated rats. 4. Contraction and half-relaxation times of the isometric twitch were significantly longer in old than in young controls. This was paralleled by an age-related decrease in the caffeine threshold of the SR. Four weeks of T3 treatment eliminated the age-related differences in both speed of twitch contraction and caffeine thresholds. V0, on the other hand, was slower in old than in young animals, both control and T3-treated, when cells with a similar MyHC composition were compared. 5. In conclusion, thyroid hormone can substantially reverse at least some of the changes that occur in ageing muscle. Further, the age-related decline in V0 in soleus fibres from control and hyperthyroid rats suggests that: (1) the identification of beta/slow myosin isoforms is incomplete; or (2) the molecular characteristics of MyHC differ between young and old age; or (3) MyHC is not the only determinant of V0.