A large, annular contrast-flash that raises thresholds for a test spot under some conditions can lower theresholds under other conditions. Incremental thresholds for a 2.5° test flash were measured upon a 10° adapting field. When an annulus, 2.5-9°, was flashed shortly before or shortly after presentation of the test spot, threshold was higher than when the annulus was not presented. When the test spot and annulus were flashed simultaneously, however, test threshold was lower with the annulus than without. This transient sensitization occurs over a limited range of annulus and adapting field intensities. Under appropriate conditions, the effect exceeds 0.5 log units. © 1980.