The parity-violating asymmetry [ital A] is calculated for quasielastic electron scattering. We use a relativistic mean field model of the nuclear matter response. A Lorentz scalar mean field (which reduces the nucleon's mass) decreases [vert bar][ital A][vert bar] by a relatively large amount. This effect must be understood before information on strange quark contributions or weak axial currents can be extracted. Alternatively, very interesting information on isoscalar convection currents could be obtained.