A flow in a rectangular channel with a step-changing cross-section was studied and the velocity fields were determined using the Laser Doppler Anemometry (LDA) method. Flows characterised by the Reynolds numbers Re = 2000, 3000, 4000 were analysed in the channel with various distances between recesses and protrusions. On the basis of the results obtained averaged velocity fields, transit stream and recirculation zone interfaces as well as fields of relative velocity fluctuation in the measuring range were determined.