The predicted and actual evolution of viscosity due to fiber orientation and concentration evolution during rheology measurements is studied in this work. The predicted transient stress response is compared to measured stress responses in prepared samples which contained non-uniform concentration and non-random orientation. The evolution of concentration and orientation towards their fully developed states occurred at different rates, resulting in a complex transient response. X-Ray computer tomography was used to determine the actual fiber morphology (orientation and concentration) profiles of samples taken at various stages of the evolution. Alternative rheology models which include the morphology effects into the stress calculation are compared and matched against the measured response.