Collagen-induced arthritis (CIA) is an animal model of rheumatoid arthritis (RA) that can be regressed with Taxol (paclitaxel), a chemotherapeutic agent. To identify structural changes that occur with involution, the synovium from naive, untreated CIA, and Taxol-treated CIA rats were evaluated by light microscopy plus transmission and scanning electron microscopy. Analysis included detailed images of vascular networks using polymeric corrosion casts. The CIA synovium was morphologically similar to human RA synovium. In CIA, the integrity of the intimal lining is lost by Type-B synoviocytes becoming highly elongated and polarized toward the joint space, resulting in non-overlapping cellular processes and the elimination of the basal lamina. In addition, the lining expanded from a width of 6-10 microns in naives to 200-250 microns in CIA due primarily to increased numbers of both Type-A and -B synoviocytes and more interstitial matrix. Vascular corrosion casts of CIA synovium illustrated a marked increase in blood vessel volume and an extensive interconnecting vascular architecture; neovascular arrays were observed to project toward the synovial surface. In Taxol-treated CIA, the synoviocyte and neovascular components reverted to the naive synovium morphology, suggesting that this agent might be useful in the therapy of RA.