The behavior and time evolution of the large-scale magnetic fields and ionospheric plasma of the dayside Venus ionosphere were studied using a one-dimensional model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for three ions (O +, O2 + H +) and the magnetic field. The CO2 + ion was included photochemically The calculated magnetic field profiles are in good agreement with observations made by the Pioneer Venus orbiter magnetometer. The magnetic field structure is found to be quasi-steady for slow changes of the solar wind dynamic pressure. The peak of the magnetic field at 165 km is maintained by the downward convection from higher altitudes. The time scale for the decay of magnetic field by the pure one-dimensional vertical diffusion/convection process is several hours unless the magnetic field flux is resupplied from the top of the ionosphere. Good agreement was obtained between the calculated and observed plasma densities for altitudes below 250 km including the electron density ''ledge'' near 190 km in magnetized ionospheres. However, the significant disagreement between the calculated and observed plasma densities at high altitudes suggests that under magnetized conditions, horizontal transport process become important in removing the plasma and/or the magnetic field even in the subsolar region. ¿ American Geophysical Union 1987