Experimental investigations using domestic pigs were carried out in order to study the subject of resorption of lyophilized Dura (Lyodura) interposed in the region of the temporo-mandibular joint after operations for ankylosis. The condyle of the mandible was partially resected on one or both sides, the joint capsule and disc excised and the mandibular fossa stripped of cartilage. Lyodura was interposed between the surfaces of the bones and intravital sequential labelling of the bones was carried out using fluorochromes. The animals were sacrificed after 18, 36, 50 and 120-130 days and the joint region excised. Undecalcified microtome (5-8 mu) and ground sections were prepared and contact microradiographs made. It was found that the Lyodura remained in place in spite of the mechanical loading until it was assimilated by a foreign body reaction and replaced by largely collagenous, endogenous connective tissue. Spaces similar to the joint cavity and lined with pseudosynovia were found after 120-130 days.