Early research on visual imagery led investigators to suggest that mental visual images are just weak versions of visual percepts. Later research helped investigators understand that mental visual images differ in deeper and more subtle ways from visual percepts. Research on motor imagery has yet to reach this mature state, however. Many authors have implicitly subscribed to the view that motor images are just weak versions of physical actions. We tested this view by comparing motor learning in variable practice conditions with motor learning in constant practice conditions when participants either physically or mentally practiced golf-putting. We found that physical and mental practice both resulted in significant learning but that variable practice was only better than constant practice when participants practiced physically. This outcome was not predicted by the hypothesis that motor imagery is just a weaker form of real-action experience. (Contains 3 figures, 1 table, and 1 footnote.)