The rod system mediated detection of test flashes, but its sensitivity was altered by excitation of the cone system by a concentric background. The magnitude of this rod-cone interaction depended on the diameter of the background field, but in different ways for different observers: large backgrounds caused greater interactions than small backgrounds in two observers, and nearly the opposite spatial pattern occurred in a third. These results were the same with both adjustment and forced-choice procedures, but increasing background illuminance increased the interaction and changed its spatial pattern. We conclude that rod-cone interaction confounds tests of spatial sensitization and contributes to interobserver differences when a red background is used to isolate the rod system. No simple physiological model seems to account for variations in spatial patterns of rod-cone interactions. © 1979.