A modified form of the BKZ elastic fluid is presented. The specific form chosen is used to describe the normal stress and shear stress responses of PMMA in two‐step torsion strain histories. Our results show that the modified form of the BKZ theory does not account for mechanical aging effects in PMMA. However, the response of mechanically conditioned PMMA to the strain histories tested is described by the modified theory. An interesting result is that the normal stress response to a two‐step strain history in which the second step strain is half the first step is predicted to be independent of the duration of the first step and identical to the response to a single‐step history at the strain level of the second step. This response is observed experimentally.