ÐThe unsteady ̄ow of blood in a straight, long, rigid pipe, driven by an oscillatory pressure gradient is studied. Three dierent non±newtonian models for blood are considered and compared. One of them turns out to oer the best ®t of experimental data, when the rheological parameters are suitably ®xed. Numerical results are obtained by a spectral collocation method in space and the implicit tr...