This paper presents a simple theory for a non-Newtonian fluid, especially the corotational Jeffreys model. Particular attention is paid to the frequency spectrum of the strain fluctuations, and through this article it is found that the Jeffreys fluid will exhibit an "onset" Reynolds number, above which the effects of the non-Newtonian nature of this fluid are felt. Because time dependent behavior of the stain-strain correlation is emphasized, this study is complementary to the molecular theory.