This paper reports the development and performance of a large scale kinetic simulation using a three-dimensional model of the terrestrial electric and magnetic fields in an effort to reach a better understanding of the ionospheric contribution to the near-Earth (x<10RE) region during quiet and slightly disturbed times. The simulation employed the Tsyganenko [1989> magnetic field model and an electric field derived from the Heppner and Maynard [1987> ionospheric potentials. For the conditions considered in this study (southward interplanetary magnetic field (IMF), &phgr;XT=20--40 kV), it was found that the cleft ion fountain plays a relatively minor role in supplying particles to the near-Earth region. The ionospheric contribution to the near-Earth proton population is significant during quiet times with the bulk of the O+ ions in the near-Earth region coming from the auroral zone upwelling region. However, the plasma mantle becomes the dominant hot proton source during more active times. Using the nightside auroral zone as a source, we launched distributions of H+, He+, and O+ ions and calculated densities, pressures, and other bulk parameters in the near-Earth plasma sheet and partial ring current. Because of the static nature of the model, ionospheric ions had very limited access to the trapped ring current, but the ions formed a reservoir of energetic particles just outside this region that in theory could act as a source for the ring current during more active times. The residence time of ions in the model is too short for charge exchange losses to become significant, and the principal loss mechanism is through the dusk flank of the magnetopause, with precipitation into the ionosphere playing a minor role. ¿ American Geophysical Union 1996