We propose a method to measure heat capacities at low temperature and in high magnetic field. The method is based on an improved version of the thermal relaxation technique and can also be used to measure the nuclear spin‐lattice relaxation timeT1and the nuclear heat capacity. As an example, we present the results of an experiment that measured the electronic heat capacity andT1of pure copper at temperatures below 1 K in a 10 T field. ©1996 American Institute of Physics.