Abstract Measurements have been performed on the mechanocaloric effect in liquid helium down to about 0.4°K. A calorimeter partly filled with 20 gram of a paramagnetic salt and 9 cm 3 of liquid helium was connected by means of a superleak to a metal bellows which was totally filled with liquid. By changing the volume of the bellows superfluid helium could be made to flow into or out of the calorimeter causing a cooling or heating respectively. Changes in temperature were of the order of a few hundredths of a degree for a flow of about 1 cm 3. The results are compared with values derived from H. London's formula making use of the specific heat data of Kramers e.a. and Wiebes e.a. The agreement is good. We found at temperatures below 0.8°K an extra heat capacity in our calorimeter as was already observed by Wiebes e.a. in their experiments on the specific heat of liquid helium (see following paper). The results of measurements above 1°K are also in good agreement with the theory.