Stress-relaxation tests were conducted in compression with nickel single crystals of various orientations and purities. The experiments were performed at five temperatures, 77°, 153°, 198°, 298°, and 350°K. The data are fit by the semilogarithmic equation, Δτ(kT/B) (In A′t+1), based on reaction rate theory. Crystals oriented to produce glide on intersecting slip planes exhibited no relaxation at the three lower test temperatures. This absence of relaxation is believed to be a result of the high activation enthalpy necessary to produce dislocation motion across the intersecting glide dislocations which would markedly reduce the rate of thermal activation. Crystals oriented to produce slip on a single set of planes had only one relaxation curve at low temperatures; no subsequent relaxation after the initial one could be induced. At higher temperatures, after relaxation had ended, new relaxation was easily initiated by increasing the stress slightly. The activated volumes obtained were almost independent of the crystalline purity and of strain, and volumes varied between 6.5×10−20 cm3 at 350°K to 0.5×10−20 cm3 at 77°K. These volumes were considerably larger and had a different temperature dependence than the activated volumes previously measured by etch-pitting methods which varied between 1.0×10−20 cm3 at 273°K to 0.2×10−20 cm3 at 77°K. The discrepancy is believed to result because the rate controlling mechanisms for dislocation motion in etch-pitting and stress-relaxation experiments are different. It is shown that during stress-relaxation dislocation-dislocation interactions are rate controlling. In the previous experiment where dislocation velocities were directly measured, dislocation-dislocation interactions are believed to be of only secondary importance. Thus, the stress relaxation test while providing a simple method for obtaining the activated volumes associated with macroscopic deformation does not seem applicable for the inference of the parameters for dislocation motion in nickel measured by etch-pitting techniques.