Abstract: Laboratory apparatus for optionally heating or cooling samples, comprising a block of one or more Peltier elements which, with one of their thermal poles, are in thermal contact with an essentially rectangular block of heat conducting metal, and with the other pole are in thermal contact with a heat exchanger, this latter being thermally insulated from the metal block. One of the outer surfaces of the rectangular metal block serves as a working outer surface for heating or cooling the samples. All outer surfaces of the metal block, with the excption of the working surface and the surface in contact with the Peltier elements, are thermally insulated. The working surface may be used for heating or cooling at will through the inversion of the direction of the alimentation current for the Peltier elements.