The effects of 4 and 8 weeks of thyroid hormone (3,5,3'-triiodothyronine, T3) treatment on skeletal muscles of young (3-6 months) male Wistar rats were investigated in the present study. In the slow-twitch soleus, contraction and half-relaxation times of the isometric twitch were significantly shorter in hyperthyroid rats than in the control group, and twitch duration was shorter in rats treated with T3 for 8 weeks than for 4 weeks. All single soleus muscle fibres from hyperthyroid rats co-expressed types I and IIA myosin heavy chains (type I/IIA fibres) or type I, IIA and IIX myosin heavy chains (type I/IIAX fibres), while only type I MyHC fibres were isolated from the controls. A significantly higher content of type IIA myosin heavy chain and fast myosin light chain isoforms was observed in soleus fibres from the 8-week than from 4-week T3 group. There was no significant difference in maximum velocity of unloaded shortening (V0) between type I myosin heavy chain fibres from controls (1.12 +/- 0.46 muscle lengths s-1, n = 48) and type I/IIA myosin heavy chain fibres from the 4-1.09 +/- 0.36 muscle length s-1, n = 33) and 8-week (1.03 +/- 0.31 muscle lengths s(-1), n = 31) groups, but type I/IIAX fibres from the 8-week T3 group had significantly higher V0 (1.56 +/- 0.10, n = 5) than type I from control and type I/IIA from hyperthyroid rats. In the fast-twitch extensor digitorum longus, neither myosin isoform composition, twitch duration nor V0 was affected by 4 or 8 weeks of T3 exposure. In conclusion, a dramatic and exposure duration-dependent change in the contractile speed of the isometric twitch and the expression of fast myosin isoforms was observed in soleus, but not in extensor digitorum longus, in response to T3 treatment. Long-term T3 treatment had relatively less influence, however, on V0 at the single cell level in spite of the dramatic increase in fast myosin isoforms.