The completeness of binocular rivalry suppression was measured by recording the fraction of a trial during which targets were perceived as alternating in their entirety, rather than blending together into a composite. It was found that the completeness of rivalry declines with prolonged stimulation. A control condition in which subjects saw the same grating targets, but without rivalry, resulted in no decrement. The phenomenon was spatial-frequency specific. The results suggest that binocular rivalry suppression is mediated by a cooperative mechanism which is susceptible to adaptation