Isometric twitches, passive electrical properties and the amounts of transverse (T) tubule system and terminal cisternae in extensor digitorum longus (EDL) and soleus muscle fibres were measured in normal rats and rats given daily injections of triiodothyronine (T3, 150 micrograms kg-1) for 15-25 days. Isometric twitches in both muscles were more rapid after the T3-treatment, particularly in soleus. Cable properties were measured using a three-microelectrode, end-of-fibre, voltage clamp technique. In order to increase the space constant of the T-tubule system, extracellular solutions were used that reduced ionic, particularly chloride, conductance. Fibre diameter was less than normal in the hyperthyroid rats. Membrane capacity, per cm2 of fibre surface, increased in both EDL and soleus muscles and there was a decrease in membrane resistance. The volume and surface area of the T-system and terminal cisternae were measured using standard morphometric techniques. Following T3-treatment the amount of T-tubule system per 100 micron3 of fibre volume, in both EDL and soleus fibres, was twofold higher than in normal fibres. The larger area of T-tubule membrane per unit volume was sufficient to account for the increase in membrane capacity. In contrast, the amount of terminal cisternae per 100 micron3 of fibre was unchanged in EDL following T3-treatment and there was only a small increase in soleus. As a consequence, the normal relationship between the T-tubules and terminal cisternae was changed in both muscles. There was an increase in the numbers of 'bare' T-tubules and an increased occurrence of diadic, pentadic and heptadic junctions between the membranes of the T-tubules and terminal cisternae. The results suggest that thyroid hormone has a differential effect on the synthesis of T-tubule and terminal cisternae membrane, resulting in a disproportionately large amount of T-tubule membrane.