A brief visual target stimulus may be rendered invisible if it is immediately preceded or followed by another stimulus. This class of illusions, known as visual masking, may allow insights into the neural mechanisms that underlie visual perception. We have therefore explored the temporal characteristics of masking illusions in humans, and compared them with corresponding neuronal responses in the primary visual cortex of awake and anesthetized monkeys. Stimulus parameters that in humans produce forward masking (in which the mask precedes the target) suppress the transient on-response to the target in monkey visual cortex. Those that produce backward masking (in which the mask comes after the target) inhibit the transient after-discharge, the excitatory response that occurs just after the disappearance of the target. These results suggest that, for targets that can be masked (those of short duration), the transient neuronal responses associated with onset and turning off of the target may be important in its visibility.