Abstract: A slide projector for interchangeable circular slides magazines comprising downwardly open slide compartments (2) is provided with a mechanism, the double function of which is to lock the slide compartments and to release the slides magazine from the slide projector and, respectively, to release the slide frames (3) in the slide compartments (2) and to couple the slides magazine to the slide projector. The mechanism comprises an eccentric plate (28) which is located on the slide projector and upon rotation moves a locking ring (5) located in the slides magazine for the slide frames (3) at the same time as the eccentric device effects the coupling between the slide projector and the slides magazine. Mechanical (36) and electric (39) safety devices render slides magazine exchange prior to completed slide change impossible and, respectively, render slide change impossible while exchange of slides magazine is going on.