The necessity of controling the nuclei content of the circuit water while conducting cavitation tests on mode! hydraulic machines is considered to be indispensable. We are presenting a description of our cavitation nuclei producing device, the so-called nuclei injector, which is based on the principle of rapid expansion of air-saturated water. An experimental study has been carried out on a transparent mode! injector which has permitted to realize the visualization of the process in the form of photographs or digitized images. This study has allowed the authors to specify the influence of injection pararneters (inlet and outlet pressures) on the dimension and the number of cavitation nuclei produced.