The invention relates to a heat exchanger, in particular a radiator for motor vehicles, comprising a black consisting of tubes and ribs and a lower and upper collection chamber, the lower collection chamber having several chambers which are separated from one another by partitions and between which a fluidic connection can be established. If necessary, by means of a displaceable actuator via connection orifices (17, 18, 19) that are located in a connection channel (16). According to the invention, the actuator is configured as a piston (30) that can be displaced axially between an open and a closed position and the cross-sections of the connection channel (16) and the piston (30) taper from the exterior to the interior in the vicinity of the connection orifices (17, 18, 19).