The axisymmetric deformation behavior of 0.9999 Cu is investigated at strain rates from 10 −4 to 10 4 s −1. The variations of the flow stress and of the mechanical threshold stress (the flow stress at 0 K), which is used as an internal state variable, with strain rate and strain are measured. The experimental results are analyzed using a model proposed by Kocks and Mecking: results at constant structure are described with thermal activation theory; structure evolution (strain and strain rate evolution of the mechanical threshold stress) is treated by the sum of dislocation generation and dynamic recovery processes. A significant result is that the athermal dislocation accumulation rate, or Stage II hardening rate, becomes a strong function of strain rate at strain rates exceeding 10 3 s −1. This leads to the apparent increased strain rate sensitivity seen in a plot of flow stress at a given strain vs the logarithm of strain rate. An explanation is proposed for the strain rate dependence of this initial strain hardening rate based on the limiting dislocation velocity and average distance between obstacles.