Treadmill-based virtual environments have the potential to allow near natural locomotion through large-scale simulated spaces. To be effective, such devices need to provide users with visual and biomechanical sensations of walking that are sufficiently accurate to evoke perception-action couplings comparable to those occurring in the real world. We are exploring this problem using a custom built, computer controlled treadmill with a 6' by 10' walking surface, coupled to computer graphics presented on wide field-of-view back projection screens. The system has the added feature of being able to apply forces to the user to simulate walking on slopes and the effects of changes in walking speed. We have demonstrated the effectiveness of this system by showing that the perceptual-motor calibration of human locomotion in the real world can be altered by prior walking on the treadmill virtual environment when the visual flow associated with self-motion is mismatched relative to biomechanical walking speed. The perceptual-motor coupling that we have achieved is sufficient to allow investigation of a number of open questions, including the effect of walking on slopes on the visual estimation of slant and visual influences on gait and walking speed.