The scanning laser acoustic microscope (SLAM) was utilized to measure the change in the propagation velocity in the transverse direction during contraction in living skeletal muscles of the frog. The SLAM was operated at 100 MHz and interferograms were produced on a CRT in real time. The images of the interferogram were processed by image-analyzer and the propagation velocity was calculated from the shift of the interference line at rest and during contraction. In all the measurements (n = 15), the velocities during contraction were clearly slower (-7.6 m/s) than at rest and this means that the transverse stiffness decreased during contraction (-2.4 x 10(7) N/m2). The decrease in the propagation velocity preceded the increase in force by 30-40 ms after stimulation, suggesting that the decrease in the transverse stiffness reflects the basic molecular change in muscle contraction.