The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, aged at 288 degrees C in argon environment for up to 2000 h was investigated. The experimental program was designed to explore the influence of prior isothermal aging on monotonic loading and unloading at various strain rates. In addition, the relaxation response and the creep behavior of specimens subjected to prior aging of various durations were evaluated. All tests were performed at 288 degrees C. The time-dependent mechanical behavior of the PMR-15 polymer is strongly influenced by prior isothermal aging. The elastic modulus increased and the departure from quasi-linear behavior was delayed with prior aging time. Stress levels in the region of inelastic flow increased with prior aging time. Furthermore, prior aging significantly decreased the polymer's capacity for inelastic straining, including the material's capacity to accumulate creep strain. Conversely, the relaxation response was not affected by the prior aging. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2956-2962, 2009