The inelastic deformation behavior of PMR-15 neat resin, a high-temperature polymer, was investigated at 260 -C. The experimental program was designed to explore the influence of strain rate on loading and unloading behaviors. In addition, the effect of prior strain rate on creep, relaxation, and recovery responses was evaluated. The material exhibits positive, nonlinear strain rate sensitivity in monotonic loading. Early failures occur before inelastic flow is fully established. The creep, relaxation, and recovery responses are significantly influenced by prior strain rate. The experimental data were modeled using the Viscoplasticity Based on Overstress for Polymers (VBOP) theory. The effects of prior aging in argon at 260 -C on the time (rate)-dependent behavior of the PMR-15 polymer were evaluated in a series of strain controlled experiments. Several of the VBO material parameters were expanded as functions of prior aging time. The resulting model was used to predict the behavior of PMR-15 subjected to various prior aging durations.