The effects of abrupt increases and decreases of light in one part of the retina on sensitivity in an adjacent part of the retina were studied under low scotopic conditions near absolute threshold. The basic finding is that a brief annular surround decreases sensitivity to test spots occurring near, the time of its onset or its offset, but that the surround increases sensitivity to test spots occurring the first few hundred milliseconds following its offset. If the annulus is presented in one eye and the test flash to the other, annulus onset reduces sensitivity, but annulus offset has almost no effect on thresholds in the contralateral eye. In fully dark-adapted observers, absolute sensitivity is somewhat greater when the test flashes are preceded by a flashed surround than when they are presented alone. These results challenge the idea that sensitivity to these stimuli is limited by an irreducible level of noise in the dark and under dim illumination. © 1982.