The dynamics of viscoelastic liquids in a thixotropic shear loop is examined. Shear start‐up and thixotropic loop experiments with an aqueous poly(acrylamide) solution were conducted under a wide range of Deborah and Weissenberg numbers. The transient stresses generated by the liquid were measured and compared with predictions of the Wagner model. An efficient linear regression algorithm, utilizing a singular‐value decomposition routine, was developed and used to extract the linear parameters of the model from dynamic‐mechanical data. The nonlinear parameter was obtained from start‐up data by Wagner's method. The dynamic response of the liquid in a thixotropic loop is characterized by several distinct features, including stress overshoot and residual stress, which are well reproduced by the Wagner model. The general form of the dynamic response function is governed by the specific combination of the Deborah and Weissenberg numbers.Overall, the results indicate that viscoelastic effects may lead to distinct hysteresis in the response function in a thixotropic loop experiment, which may be erroneously interpreted as a rheological thixotropic effect.