The mechanical parameters of the periodontal ligament (PDL) in rat specimens were investigated in a combined experimental and numerical approach. Tooth mobility of the rat mandibular first molar was measured in vitro using a high precision experimental set-up. Finite element models (FEM) were developed, based on histological sections of the measured specimens, to simulate tooth mobility numerically under the same force systems as used in the experiment. Force/deflection curves from the measurements showed a significant non-linear behaviour of elastic stiffness of the PDL. A bilinear material parameter set was assumed to simulate tooth deflections. The numerical force/deflection curves were fitted to the experimental curves by repeatedly calculating theoretical tooth deflections and varying the parameters describing the non-linearity. Mean values of E1 = 0.15 MPa, E2 = 0.60 MPa and an ultimate strain of epsilon12 = 6.3 per cent were derived for the elastic behaviour of the rat PDL. Comparing fresh specimens and those frozen in a 0.9 per cent saline solution, differences between the measurements were significant. Using the agent, Periston, for freezing significantly reduced the deviation. The results indicated that strains in the PDL with a maximum of 14 per cent at the furcation were 10(4) times higher than strains in the bone, while the variability of stress values in both PDL and bone was not significant.