The effects of different combinations of segmented intrusion arch wires and anchorage units on the relative vertical positions of anterior and posterior teeth in the lower arch were demonstrated in four nongrowing baboons. Forces of between 90 and 100 g were delivered by the intrusion arch wires to the four lower incisor teeth in each animal over a period of 5 months. The dental and skeletal changes occurring during that period were assessed from lateral cephalometric radiographs. Lower incisor intrusion, determined by the vertical movement of an internal reference point, was demonstrated in each of the animals. However, the actual effects of the mechanics on the relative anterior and posterior vertical tooth positions, and consequently on the height of the lower face, were found to depend largely on the magnitude of the reactive moments acting on the anchorage units. It was suggested that in general, for a given intrusive force, the further forward the center of resistance is positioned in the anchorage unit, the smaller the reactive moment will be and the more incisor intrusion one might reasonably expect to achieve during arch leveling.