Previous studies of the periodontal ligament (PDL) have applied high forces to the dental units to examine the stress-strain behavior of this soft tissue. In this study, cadaveric specimens of mandibular premolars from 2 young adult and 2 elderly adult donors were tested to determine the biomechanical behavior of the PDL over an orthodontic force range. Transverse specimens were prepared from 9 premolars and subjected to loading in intrusion and extrusion. Stress-strain curves for both loading directions had distinct toe and linear regions, demonstrating nonlinear behavior of the PDL. The average linear shear modulus was higher for intrusion than for extrusion. The toe extrusive modulus was higher for the young group, and extrusive toe size was larger for the elderly group. In extrusion, the average modulus was higher for the cervical margin and the apex regions than for the midroot regions. The size of the toe region was smaller for intrusion than extrusion. The results indicate age-dependent, location-dependent, and load-direction-dependent nonlinear properties of the human PDL and suggest that analytical computer simulations of orthodontic tooth movements might benefit from incorporating the nonlinear material properties of the PDL. Copyright © 2002 by the American Association of Orthodontists.