Incremental thresholds were measured against four kinds of background light that had identical effects on rods: blue or red light entering the pupil near its edge or near its center. The match of scotopic luminance for each subject was based on his own absolute rod threshold for the two colors. Data from dark adaptation curves and the Stiles-Crawford effect show that only the rod system could detect the test flash. Yet, the threshold was either 0.3 or 0.5 log units higher against the red background than against the scotopically matched blue background, depending upon whether the red background light entered the pupil near its edge or near its center, respectively. The rods were shown to be indifferent to the loci at which the background light entered the pupil. The incremental threshold rose by more than a log unit when the test flash was presented at the same time as the change of locus at which the red background light entered the pupil. It follows that excitation of cones can prevent detection of a test flash by the rod system. © 1974.