A method is described for detecting rapid changes in sarcomere length at different points along a muscle specimen during stimulation. Reproducible differences in behaviour are observed at different positions along the length of the muscle both during contraction and during relaxation. By summing individual observations made at points closely spaced along the specimen it is found that in tetanus at 6 degrees C the average sarcomere contraction is about 5% for an initial length of 2.6 mum; a maximum rate of contraction of about 4 nm.ms-1 (0.16% ms-1) is reached and maintained for some 10 ms during the development of tension. In twitch the corresponding values are 4% and 3 nm.ms-1 (0.12% ms-1).