Besides luminance change, the pupil responds to changes in spatial pattern, color content and target motion. Our experimental results show that transient pupillary constriction can also be elicited by dichoptically viewing a change in stereoscopic stimuli composed of dynamic random-dot stereograms from an initially flat surface to a stationary sinusoidal grating shown in depth. On the other hand, monocular viewing of these stimuli produced no obvious pupillary response. This indicates that the pupillary response in the dichoptic experiment was induced by the stereo information rather than by any change in the monocular stimuli. This finding presents a novel approach for the investigation of stereo perception that can also be applied in the clinical environment.