Entry of fluorescent dyes into the sarcotubular system of the frog muscle.

1. A fluorescence microscope was used in order to detect whether fluorescent dyes can quickly diffuse into and out of certain parts of single muscle fibres of the frog.2. Several fluorescent dyes entered a system arranged at the centre of each I-band without entering the main part of the fibre.3. The dyes quickly diffused out of the fibres on washing, becoming invisible within a few minutes.4. In a hypertonic solution the fluorescent striations were more conspicuous and faded more slowly.5. It is concluded that the dyes have access to some component of the triads.6. The ;dye space' measured with Lissamine Rhodamine B200 was 1-2% of the fibre volume, and the implication of this value is discussed.7. From an analysis of the time course of entry or exit of the dye it is suggested that the principal resistance to radial diffusion of the dye is distributed along the tubules rather than at the mouths of the tubules.