The process of grain growth in ultrafine-grained nickel was studied using differential scanning calorimetry. Samples of nickel, with mean grain sizes in the range from 100 to 400 nm, were prepared by equal-channel angular pressing, high pressure torsion or combinations of both. It is shown using differential scanning calorimetry that grain growth in these structures begins at temperatures in the range from 500 to 700K and the measured activation energy for grain growth is close to the activation energy for grain boundary self-diffusion. There was also evidence for a decreasing activation energy in the samples prepared by high pressure torsion.