A calorimeter for measuring nuclear heating in reactors is described. The calorimeter is of simple construction and high sensitivity. Calorimeters of this design were used to measure the nuclear heating in magnesium and bismuth in an experimental facility of the ORNL Graphite Reactor. Gamma radiation gives the most important contribution to nuclear heating for these two elements. Self-shielding is estimated, and the consistency between the measurements for bismuth and magnesium is demonstrated.