We modeled the chemical and physical processes taking place in the nightside ionosphere of Venus by solving the one dimensional coupled continuity and momentum equations for 12 ion species [CO+2, O+2, O+, H+, NO+, CO+, N+2, N+, He+, C+, O+(2D) and O+(2P)>. We investigated the relative importance of the two major processes responsible for maintaining the nightside plasma densities: atomic ion transport from the dayside and impact ionization due to energetic electron precipitation. We compared our model calculations with electron density and ion composition observations obtained by instruments aboard the Pioneer Venus Orbiter during both high and moderate solar cycle conditions. These studies lead us to conclude that day-to-night atomic ion transport is dominant during high solar activity, while during moderate solar activity conditions the combined effects of the electron precipitation and reduced day-to-night ion transport are responsible for maintaining the nightside ionosphere of Venus. ¿ American Geophysical Union 1995