Conditions under which duration-intensity reciprocity holds for acuity performance were investigated. Reciprocity fails to hold for the resolution of a Landolt C at 40 mL X msec when it is immediately followed or preceded by a 2-sec flash of 1 mL: performance then increases with exposure duration. Reciprocity holds when the interval between target and flash is increased to 1.5 sec. When the target is superimposed on the adapting field, reciprocity is found, but the critical duration is considerably shorter. Results are discussed in terms of recent theorizing which attributes masking by light to effects of brightness summation. The existence of an additional interference effect is indicated. The nature of this interference is discussed, with emphasis on the close similarity found between results for forward and backward masking by light. (28 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)