The volume of the T-system and its association with the sarcoplasmic reticulum in slow muscle fibres of the frog.

1. A study has been made of the T-system and sarcoplasmic reticulum (SR) in slow muscle fibres of the frog, Rana temporaria.2. The size of the T-system was measured by an autoradiographic method, using tritium-labelled albumin as a marker. Its volume, expressed as a fraction of that of the fibre, was found to be 1.8 x 10(-3), as compared with a figure of 3.9 x 10(-3) for the T-system in a twitch fibre.3. The spatial distribution of the T-tubules, and their association with the SR, was studied with the electron microscope, employing ferritin and the enzyme peroxidase as markers. The observations show (a) the tubules form a three dimensional, rather than transverse, network and (b) the area of triadic (and diadic) contact with the SR is 5-10 x smaller than in a twitch fibre.4. The possibility that the T-system and SR of the slow fibre participate in linking membrane excitation with contraction is discussed in the light of these findings.