Four experiments were performed to assess the effects of ON channel blockade with the glutamate analog 2-amino-4-phosphonobutyrate (APB) on brightness and contrast perception in monkeys. In Experiment 1, we demonstrate that stimuli brighter than background (incremental stimuli) appear less bright following ON channel blockade. This decrease in brightness is not enough to account for the previously observed threshold increase for detection of incremental stimuli following APB administration (Schiller et al., 1986; Dolan & Schiller, 1989). Experiment 2 examines the role of the ON and OFF channels in the interaction between local contrast and apparent brightness. The phenomenon of simultaneous contrast was examined under normal conditions and following APB administration. We find that even following ON channel blockade, the brightness of a stimulus is determined primarily by its contrast with its immediate background. This indicates that the lateral processes involved in simultaneous contrast can operate even when one channel has been compromised. In Experiment 3, we examined the role of the ON channel in detection of stimuli that appear by virtue of changes in background vs. foreground luminance. We find that the ON channel selectively conveys information pertaining not only to the temporal nature that defines the stimulus as incremental but also to the spatial features that define it as incremental. In Experiment 4, we test the hypothesis that incremental and decremental temporal luminance ramps are differentially processed by the ON and OFF channels to a higher degree than are step-luminance changes. We find that the detection of incremental ramps is no more affected than is the detection of incremental steps following APB administration.