Previous studies have demonstrated that both visual and proprioceptive feedback influence motor control. The relative contributions of these sensory modalities to the on-line computation of body position--that is, the body schema--remain unclear. We report a study designed to explore the roles of vision and proprioception in motor planning. The task required subjects to judge if a pictured stimulus was a right or left hand; stimuli included pictures of a right or left hand in a palm up or palm down position and in six different angular rotations (0 degrees , 60 degrees , 120 degrees , 180 degrees , 240 degrees , 300 degrees ). Each subject was tested with his/her right hand palm down and palm up. There were three conditions: a "control" condition (real hand in view), a "fake hand" condition (fake hand in view, real hand out of view), and a "proprioception" condition (no fake hand, real hand out of view). We found that proprioceptive input (that is, the subject's "felt position") had a significant influence on mental rotation whereas the visually perceived posture of the hand did not. We suggest that, at least under some circumstances, proprioceptive inflow may represent the dominant sensory input to the on-line representation of the body in space.