To study the mechanism of crack initiation in the fatigue of amorphous polymers, flexural fatigue tests have been performed on injection molded samples (ISO 1 standard 4 mm thick 10 mm large) of polystyrene homopolymer (PS), at a frequency of 10 Hz. Two types of fatigue tests were performed : i) Monosequential Fatigue Test (MFT) in which cycling loading is not interrupted; ii) Polysequential Fatigue Test (PFT) during which fatigue periods alternate with rest periods. The number of cycles to rupture has been determined for various values of the strain amplitude, temperature and duration of rest periods. Two kinetics regimes were observed : i) at temperature higher than 75 degrees C, the thermal regime is quasi adiabatic and failure results from polymer softening. In this regime, rest periods have a strong stabilizing effect since they limit self heating; ii) At temperatures lower than 75 degrees C, the thermal regime is quasi isothermal and fracture results from crazing followed by brittle rupture. Rest periods [on the order of half fatigue lifetime] have no effect on the number of cycles to rupture. The Influence of temperature here is considerably lower than in the "adiabatic" regime. The endurance limit values epsilon(infinity) are close to the critical strains for crazing epsilon(C) in static conditions. A kinetic model based on the above observations is proposed for strain amplitudes close to the endurance limit (2 epsilon(infinity) greater than or equal to epsilon greater than or equal to epsilon(infinity)). This model reasonably fits the experimental data.