The spatial distribution of acid membrane organelles and their relationships with normal and vacuolated transverse tubules has been studied in living frog skeletal muscle fibres using confocal microscopy. Acridine orange (AO) was used to evaluate acid compartments, while a lipophilic styryl dye, RH 414, was employed to stain the membranes of the T-system. AO accumulated in numerous spherical granules located near the poles of nuclei and between myofibrils where they were arranged in short parallel rows, triplets or pairs. AO granules could be divided into three groups: green (monomeric AO), red (aggregated AO), and mixed green/red. As demonstrated by lambda-scanning, most granules were mixed. Double staining of muscle fibres with AO and RH 414 revealed almost all AO granules located near the transverse tubules. Vacuolation of the T-system was induced by glycerol loading and subsequent removal. The close juxtaposition of AO granules and the T-system was preserved in vacuolated fibres. The lumens of vacuoles did not accumulate AO. It is concluded that AO granules represent an accumulation of AO in lysosome-related organelles and fragmented Golgi apparatus and a possible functional role of the spatial distribution of such acidic compartments is discussed.

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