The present invention is concerned with a control device for medium flows for a regenerative heat exchanger. The device consists of a cell (7) divided into compartments (8) and acting as a heat exchanger, and at the ends of the cell, there are control means (11, 12) for guiding the medium flows alternatingly into the different compartments (8) of the cell (7). In prior art, unreasonably high capacities of actuating devices have been required for controlling the flows of media, and the control devices have increased the length of the heat exchanger considerably. These drawbacks are avoided if the control means consist of a slide (11, 12) displaceable transversely in relation to the partition walls (9) of the cell, the said slide (11, 12) being provided with openings (13) and with blocking members (14) between the openings for the purpose of alternatingly opening and closing connections between medium flow ducts (2, 3, 5, 6) and compartments (8) in the cell.