X-ray diffraction experiments were performed on rat soleus and psoas muscles using synchrotron radiation from a multipole wiggler. The X-ray diffraction patterns were recorded at a time resolution of 16.7 ms with an X-ray image intensifier and a CCD video camera. A bundle of 10-15 muscle fibres was activated by photorelease of 2.2 mM ATP from 7 mM caged-ATP at 15 degrees C. In soleus muscle fibres in the presence of 40 microns Ca2+, the tension developed with a half-time of 0.13 s. The intensities of the (1,1) equatorial reflection and the actin layer-lines at 1/36 nm-1 and 1/5.9 nm-1 decreased from their rigor values in the first 16.7 ms frame to a level higher than in the resting state, and then remained unchanged during the tension development. The intensity of the 14.5 nm meridional reflection increased as the tension developed. These results show that after the ATP release some myosin heads are stereospecifically attached to actin well before tension development, and suggest that their number does not change markedly on contraction.