1. Transverse electrical field stimulation (50 V cm-1, 2 ms duration) of mechanically skinned skeletal muscle fibres of the rat elicited twitch and tetanic force responses (36 +/- 4 and 83 +/- 4 % of maximum Ca2+-activated force, respectively; n = 23) closely resembling those in intact fibres. The responses were steeply dependent on the field strength and were eliminated by inclusion of 10 microM tetrodotoxin (TTX) in the (sealed) transverse tubular (T-) system of the skinned fibres and by chronic depolarisation of the T-system. 2. Spontaneous twitch-like activity occurred sporadically in many fibres, producing near maximal force in some instances (mean time to peak: 190 +/- 40 ms; n = 4). Such responses propagated as a wave of contraction longitudinally along the fibre at a velocity of 13 +/- 3 mm s-1 (n = 7). These spontaneous contractions were also inhibited by inclusion of TTX in the T-system and by chronic depolarisation. 3. We examined whether the T-tubular network was interconnected longitudinally using fibre segments that were skinned for only approximately 2/3 of their length, leaving the remainder of each segment intact with its T-system open to the bathing solution. After such fibres were exposed to TTX (60 microM), the adjacent skinned region (with its T-system not open to the solution) became unresponsive to subsequent electrical stimulation in approximately 50 % of cases (7/15), indicating that TTX was able to diffuse longitudinally inside the fibre via the tubular network over hundreds of sarcomeres. 4. These experiments show that excitation-contraction coupling in mammalian muscle fibres involves action potential propagation both transversally and longitudinally within the tubular system. Longitudinal propagation of action potentials inside skeletal muscle fibres is likely to be an important safety mechanism for reducing conduction failure during fatigue and explains why, in developing skeletal muscle, the T-system first develops as an internal longitudinal network.