Chemically skinned single fibres from soleus (slow) and tensor fascia latae (fast) muscles of the euthyroid rat and guinea-pig generated maximal isometric tensions in a narrow range of 125 - 150 kN/m2 at a temperature of 25 °C and a sarcomere length of 2.75 μm. The maximum velocity of shortening (Vmax) of the tensor fascia latae (TFL) muscle fibres from the euthyroid rat (12.35 +/- 0.95 LoS-1 - Fibre lengths per second) and guinea-pig (9.93 + 1.14 LOS-1 ) were 2-3 fold higher (P < 0.05, unpaired t-test) as compared to the soleus muscle fibres of the euthyroid rat (5.61 +/- 0.54 LoS-1 ) and guinea-pig (3.92 + 0.35 LoS-1 ) respectively. This was consistent with previous (e.g. Reiser et al., 1985a, b) results on slow and fast muscle fibres. Storing bundles of soleus or TFL muscle fibres from euthyroid rats and guinea-pigs for short (weeks) or long term (months) resulted in significant (P < 0.05, unpaired t-test) reductions in both the maximum isometric tension (Po) and Vmax of stored single fibres in comparison to fresh single fibres. Therefore, all subsequent experiments were done using fresh single fibres. The maximum isometric tension of chemically skinned single fibres from the soleus of mildly hyperthyroid animals (rats), was lower than that generated by equivalent fibres from mildly hypothyroid animals (127.12 +/- 5.87 kN/m2vs. 169.02 +/- 6.62 kN/m2, P < 0.05, unpaired t-test). There was a corresponding reduction in tension production in TFL fibres comparing hyperthyroid with hypothyroid animals (121.30 +/- 6.80 kN/m2 vs. 142.73 +/- 6.38 kN/m2, P < 0.05). On the other hand, Vmax of single fibres from soleus muscles of hyperthyroid rats was higher than the hypothyroid counterparts (6.58 +/- V 0.32 LoS-1 vs. 5.52 +/- 0.27 LoS-1 , P < 0.05, unpaired t-test). Similarly, Vmax of hyperthyroid single TFL fibres was higher than the hypothyroid counterparts (13.81 +/- 0.50 LoS-1 vs. 11.18 +/- 0.65 LoS-1 , P < 0.05). These differences between the hyper- and hypothyroid results - within each muscle type - were observed in the same type of fibre as identified histochemically. These were the slow oxidative (SO) fibre type from the soleus and the fast glycolytic (FG) fibre type from the TFL muscles. It is postulated that the differences in PO and Vmax were due to alterations in the isoforms of the myosin molecule and more specifically to the myosin light chains. Mild dysthyreosis showed that single muscle fibres of the same histochemical type can generate heterogeneous tension/pCa (T/pCa) relationships.