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The non‐linear response of the tooth to external stresses is due to the interaction of the fibrous, cellular and vascular elements of the supportive tissues. In this study the relaxation of the central incisors after the suden removal of axial forces of 2.5 Newton was investigated in three monkeys. The displacement‐time curve was analysed using a technique of exponential curve fitting. Analysis shows that the periodontium is viscoelastic in nature and may be represented by three and possibly five Voigt elements. Comparison of the upper and lower incisors indicate that while the same basic processes are active in both mandible and maxilla they are different in magnitude. In the case of the fastest acting Voigt element the response is more rapid for lower teeth than for upper ones, corresponding either to less damping or a stronger spring constant. This difference in magnitude is presumably due either to differences in the architecture of the periodontium and alveolar bone or to the local blood supply. The authors are indebted to Dr. M. Braden of the London Hospital Medical College for his advice and guidance. Thanks are due also to members of the Department of Medical Physics of the University College Hospital, particularly Mr. J. S. Clifton for his encouragement, Mr. M. Conway and Mr. R. Favre for the construction of the force applicator and Mr. K. J. Cassell for his assistance.

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