1. The course of cone dark adaptation after exposure to a strong bleaching light depends on the size of the bleached region. Threshold for brief, tiny test flash centred in the bleached region is elevated more, and recovery is retarded by a small bleach. This effect has its parallel in the sensitization effect observed with steady backgrounds. 2. Previous results, that a similar sensitization effect is not observed in rod dark adaptation, are confirmed. 3. This sensitization effect in cone dark adaptation does not transfer binocularly, and is unaffected by pressure blinding during the bleaching exposure. 4. Threshold following a small bleach may be lowered by adding a steady annular background to the region surrounding the bleached patch. Conversely, bleaching the area surrounding a small, steady background can lower threshold for a test flash centred on the background. 5. These interactions between backgrounds and bleaches may be explained if bleaches produce long-lasting signals from neurones in the bleached area, which then lead into a spatially opponent stage of processing. 6. It is likely that the persisting signals come from the cone receptors, since the Bunsen-Roscoe Law (intensity-time reciprocity) holds for small bleaches as well as large, for durations up to about 3 sec.