AJLynn

Could you elaborate on that boxed light setup? Where is it? are you talking about a rectangular box around the entire scene with a self illuminating interior surface providing you primary light source? I don't think mental ray would be happy with that sort of thing.

Try one where you have the light portals in the windows (arrow points in) and there is no geometry surrounding the scene and no lights except an mr sun and mr sky, FG is turned on and you use mental ray exposure control (the indoor sunlight preset is a starting point). The window glass should be Arch&Design thin geometry glass, which will be dealt with correctly by the renderer. To keep things simple use a matte light gray A&D material for everything else in the scene and see if you can get the lighting to work.