The elastic moduli of MgSiO3 perovskite have been measured on both single-crystal and polycrystalline samples by Brillouin spectroscopy at ambient conditions. The aggregate elastic moduli calculated from measured single-crystal elastic moduli and from the average polycrystalline velocities are in excellent agreement. This convincingly demonstrates that accurate values of elastic moduli can be obtained for polycrystalline samples of dense anisotropic materials by Brillouin spectroscopy. Our new values of the bulk modulus (KS = 253(3) GPa) and shear modulus (¿ = 175(2) GPa) are ~4% and ~1% lower than commonly accepted values for end-member MgSiO3 perovskite (KS = 264 GPa, ¿ = 177 GPa). These lower elastic moduli suggest that the pressure, temperature, and compositional dependence of elastic moduli and wave velocities for magnesian silicate perovskite must be reevaluated in order to obtain reliable constraints on the mantle chemistry and thermal structure.