The sensitivity of the rod system to a small test flash is reduced when a concentric background excites cones. For most observers, this rod-cone interaction is greater on small backgrounds than on large backgrounds. However, some observers show nearly equal interaction on both small and large backgrounds. For all observers, the magnitude of interaction on large backgrounds increases with increased background illuminance. Thus, increases of background illuminance can reduce the differences among observers, but some differences remain. The residual differences may represent fundamental differences among observers or the effect of some unexamined parameter. © 1981.