Strain rate change tests were performed during low cycle fatigue of polycrystalline copper using plastic strain as the control variable. The evolution of dislocation interactions was observed by evaluating the activation area and true stress as a function of cumulative plastic strain. Activation areas at each of three plastic strain amplitudes, โฌe p /2 = 0.2, 0.4, and 0.6%, have initial values of approximately 2000b 2 which decrease to 600b 2 during cyclic loading to saturation. This observation suggests a transition from forest dislocation cutting to increasing contributions of cross-slip as the predominant rate-controlling mechanisms of dislocation motion. Haasen plots of normalized inverse operational activation area (b 2 /โฌa) for specimens cycled to saturation exhibit a deviation from linearity similar to that observed for monotonic deformation. This nonlinearity corresponds to a failure of the Cottrell-Stokes law that correlates with the development of characteristic dislocation structures during cyclic deformation. Tests performed at various stresses at saturation reveal a linear dependence of b 2 /โฌa on true stress. The athermal stress, s b =86.5 MPa, measured at saturation by extrapolating the activation area data compares favorably with the value determined from a Bauschinger analysis, s b =80 MPa, at a plastic strain amplitude of 0.6%. In addition, athermal stress values vary with plastic strain amplitude as expected, resulting in a constant value of approximately s b /s=0.5.