The purpose of this thesis was to design, construct and test a system to measure sarcomere length by loser diffraction in striated muscle ' in vitro'. I designed a muscle chamber to allow illumination of all points on a muscle, transmission of the diffracted light, temperature control and stimulation. Control software was developed to detect the position of diffracted orders. Force, muscle length and sarcomere length data were integrated. To test the system, force, muscle length and sarcomere length were measured in frog semitendinosus muscles during passive stretch, fixed-length tetanic (isometric) contractions, and contractions during sinusoidal muscle length changes (eccentric). Sarcomere length changed linearly with muscle length during passive stretches from 2.3 to 2.7 [mu]m while force increased curvilinearly to 0.14 mN. In isometric and eccentric contractions sarcomere length shortened from 2.6 to 2.3 [mu]m, but force was higher in the eccentric (0.14 mN) than in the isometric contractions (0.12 mN).