A new method for measuring the width of the periodontal ligament (PDL) in orthodontically treated rats has been presented. For the present experiment the first upper molar in twelve young adult Spraque-Dawley rats were moved buccally with forces up to 700 mN. The appliance used was modified according to Kvam (13) and Rygh (32). After 24 h the rats were sacrified and the sculls were embedded in carboxymethylcellulose and frozen. In a predetermined area the surfaces of the specimens were stained with silver-nitrate and photographed. Some sections were also collected on tape and stained with hematoxylin and eosin.The width of the PDL decreased with increasing forces following a logarithmic function. When the PDL was compressed more than 30% on the buccal side and 40% on the palatal side, red areas indicating extravasated blood were found in the pressure zones. Bending of the buccal alveolar plate was associated with strong orthodontic forces. The contralaterals to the orthodontically treated teeth were also affected.The method described seems to be the first attempt to systematically correlate the compression of the PDL to the strength of the orthodontic force and to allow a simultaneous evaluation of histochemical and histological changes.