An integrating x-ray area detector that operates on the basis of laser-stimulated luminescence was used in a diffraction study of muscle contraction. The area detector has a dynamic range of 1 to 10(5), a sensitivity about 60 times greater with approximately 1/300 as much fog background as x-ray film. It is erasable and reusable but, like film, can integrate at a practically unlimited counting rate. The high sensitivity and wide dynamic range of the detector resulted in a sufficient reduction in the exposure time to make possible the recording of a clear x-ray diffraction pattern, with up to 2.0-nanometer axial spacing, from a contracting frog skeletal muscle in as little as 10 seconds with synchrotron radiation. During the isometric contraction of the muscle, most of the actin diffraction lines increased in intensity without noticeable changes in their peak positions. Changes also occurred in diffraction intensities from the myosin heads. The results indicate that during contraction the structure of the actin filaments differs from that in the rigor state, suggesting a possible structural change in the actin subunits themselves; the myosin heads during contraction retain the axial periodicity of the myosin filament and become aligned in a more perpendicular manner to the actin filaments.