The contribution of local blood flow to the buffering properties of the periodontal ligament was examined in 15 anesthetized rabbits. The position of the tooth was recorded by an ultrasonic transit time technique, and local vascular pressures and volumes were altered by electrical stimulation of the cervical sympathetic nerves. The unloaded tooth was shifted into a slightly intruded position by nerve stimulation, and from this position, intrusion movements evoked by 5- to 10-g loads were found to be 10—15% smaller than before the stimulation. Tooth mobility was unaffected by 20—30 mm Hg reductions in mean arterial pressure, but similar decreases in mobility as evoked by nerve stimulation were observed when the pressure was brought down to 10—25 mm Hg. This procedure also produced about the same intrusive shift of the unloaded tooth. Most likely, the sympathetic nerve activity decreased tooth mobility by causing a fall in periodontal vascular pressures, thereby moving the tooth into a position where it was more resistant to further, load-induced intrusion.