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1. When the tonicity of Ringer is increased above 2-5 times normal and a single fibre stimulated externally, the large, early birefringence signal preceding twitch tension (Baylor & Oetliker, 1975, 1977 a,b) is sufficiently reduced and delayed so as to reveal a small but distinct signal ('1st component") preceding it. For an average-sized fibre, the deltaI/I of the 1st component was (minus) 1 to 2 x 10-5. 2. The time course of the 1st component superimposed with the surface action potential simultaneously recorded by an internal micro-electrode. The polarity of the 1st component reversed with compensation. 3. From these characteristics, the 1st component is thought to arise from a small change in optical retardation of the surface membrane due to the action potential. 4. When a fibre was impaled with two micro-electrodes, retardation changes accompanying small hyperpolarizing and depolarizing current steps were detected. In some cases the polarity of the observed signal was opposite in sign to that expected for a retardation change only from the surface membrane. 5. Because the anatomical orientation of the T-system appears to be primarily transverse rather than longitudinal, these signals of opposite polarity are probably, on balance, due to retardation changes from the membranes of the T-system. 6. The possible origin of the large birefringence signal preceding contraction is discussed.