Abstract: The variation of wave velocity before rupture of samples composed of rocks with different properties was studied by the ultrasonic impulse method. The samples were rectangular plates of marble, in whose centre circular disks of marble, gabbro, granite and sandstone were inlaid. The experiments show that when the samples rupture the cracks extend through the rock disks with low strength, but round the rock disks with high strength. For the first group of samples the wave velocity in the inlaid rock disks varies visibly before rupture, while for the second group the wave velocity in the inlaid rock disks does not vary very much. Measurements of wave velocity through the interface between two rocks show that the wave velocity begins to decreas when the compression stress reaches 60% of rupture stress of the sample. The tendency of decrease of wave velocity before rupture of the samples is not monotonous, but it undulates. This makes clear that the heterogeneity of the medium can produce important influence on the law of wave velocity variation before rock rupture.