We report on the measurement of the leptonic and hadronic cross sections and leptonic forward-backward asymmetries at theZ peak with the L3 detector at LEP. The total luminosity of 40.8 pb⁻¹ collected in the years 1990, 1991 and 1992 corresponds to 1.09·10⁶ hadronic and 0.98·10⁵ leptonic Z decays observed. These data allow us to determine the electroweak parameters. From the cross sections we derive the properties of the Z boson: M_Z=91195 ± 9 MeVΓ_Z=2494 ± 10 MeV Γ_had=1748 ± 10 MeVΓ_ℓ=83.49 ± 0.46MeV assuming lepton universality. We obtain an invisible width of Γ_inv=496.5 ± 7.9 MeV which, in the Standard Model, corresponds to a number of light neutrino species of Nv=2.981 ± 0.050. Using also the three leptonic forward-backward asymmetries and the average tau polarization, we determine the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons to be: g¯ℓv=−0.0378^+0.0045_−0.0042 g¯ℓA=−0.4998 ± 0.0014. Within the framework of the Standard Model, and including our measurements of the Z→b¯b forward-backward asymmetry and partial decay width, we derive an effective electroweak mixing angle of sin²¯θ_W=0.2326 ± 0.0012 . We obtain an estimate for the strong coupling constant, α_S=0.142 ± 0.013 and for the top-quark mass,m_t =158⁺³²₋₄₀ ± 19(Higgs) GeV, where the second error arises due to the uncertainty in the Higgs-boson mass.Facultad de Ciencias Exactas