We theoretically study the upper critical magnetic fields at zero temperature in a quasi-twodimensional (Q2D) superconductor in the parallel and perpendicular fields, H c2 (0) and H ⊥ c2 (0), respectively. We find that H c2 (0) ≈ 0.75 |dH c2 /dT |T c Tc and that H ⊥ c2 (0) ≈ 0.59 |dH ⊥ c2 /dT |T c Tc, where |dH c2 /dT |T c and |dH ⊥ c2 /dT |T c are the corresponding Ginzburg-Landau slopes of the upper critical magnetic fields. Our results demonstrate the breakdown of the so-called effective mass model in Q2D case and may be partially responsible for the experimentally observed deviations from the effective mass model in a number of layered superconductors, including M gB2.