A full thermal activation rate theory for dynamic strain aging is developed for the case where a single rate dependent strengthening mechanism controls dislocation motion in a material (e.g. solute diffusion). The analysis shows that negative strain-rate sensitivity (SRS) cannot be obtained within such a framework, a conclusion previously reached by Hahner [Hahner P. Mater Sci Eng A 1996;207:208]. However, the SRS can be greatly reduced over a range of strain rates, making the inverse behavior more accessible by other mechanisms. In addition, the aging mechanism naturally gives rise to an instantaneous positive SRS and stress relaxation behavior under strain-rate jump conditions, putting the concepts advanced by McCormick [McCormick PG. Acta Metall 1988;36:3061; Estrin Y, McCormick PG. Acta Metall Mater 1991;39:2977] on a quantitative footing. The results here set the stage for subsequent work wherein consideration of multiple strengthening mechanisms (solute and forest hardening) operating together can predict negative SRS in quantitative agreement with data in Al-Mg alloys. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.