The optical rotation signal of a single frog skeletal muscle fibre was recorded and its properties were examined. To reduce movement of the fibre associated with its excitation, fibre was immersed in Ringer solution made of 90-95% D2O, stretched, and lightly pressed from above with a cylindrical lens. After these procedures no mechanical movements were recognizable under the dissection microscope, and the optical rotation signal appeared consistently as a brief transient increase in dextrorotation with a duration of about 23 ms. The shape and polarity of the signal did not change among preparations and remained unchanged when the direction of impulse conduction along the fibre was reversed. The influence of the birefringence change associated with excitation was found to be small in most preparations. Ryanodine (10 micron) reduced the amplitude of the signal to about half that of the control, and increased the duration slightly. Nitrendipine (10 micron) reduced the duration of the signal to about half that of the control, and reduced the amplitude to about two-thirds of the control. The effects of the chemicals took place within 10 minutes and remained constant for the periods examined. The intracellular action potentials were not appreciably altered by these chemicals. It is concluded that the optical rotation signal of the single muscle fibre reports the molecular conformational change of proteins during the early stage of the contraction process.