An ultrastructural comparison was made between the transverse tubular systems of the extensor digitorum longus (fast twitch) and the soleus (slow twitch) muscles in the rat. Quantitative measurements were made of the volume fraction, surface to volume ratio, surface density and foot density of the T system membrane. The values for the volume fraction and surface density were approximately 50% higher in the fast twitch (extensor digitorum longus) than the slow twitch (soleus) muscle. The dimensions of the tubules and hence the surface to volume ratio was the same in the two muscles. There was no difference in the spacing of the feet, so the reported three to fivefold difference in charge movement cannot be associated with a difference in foot density. Material lying within the lumen of the T tubules was observed in the majority of tubule profiles. This material, not hitherto described, was particulate or, more commonly, laminar in form.