Magnetometer measurements on the Pioneer Venus Orbiter, (PVO), have shown the presence of large-scale steady horizontal magnetic fields in the dayside ionosphere of Venus during episodes of high solar wind dynamic pressure, Psw. These horizontal magnetic fields are expected to inhibit plasma diffusion and modify the plasma distribution from a diffusive equilibrium type profile to a depressed type profile. We have analyzed electron and ion density profiles measured by the electron temperature probe and ion mass spectrometer for several orbits having large scale horizontal magnetic fields. We find that the observed density profiles are nearly similar to those expected under diffusive equilibrium, thereby suggesting that these fields perhaps do not inhibit diffusion. This observation has a major consequence for Mars ionosphere where the observed electron density profiles are of depressed type and have been explained on the basis of horizontal magnetic fields measured by the MAG/ER experiment on Mars Global Surveyor, spacecraft.