Fibronectins (FN) regulate cell migration, proliferation, and matrix formation during tissue injury. In humans, up to 20 different FN isoforms are generated by alternative splicing in three regions called EIIIA, EIIIB, and V, which have been implicated in the process undergoing wound healing and embryonic development. Specifically, EIIIA- and EIIIB-containing isoforms have been implicated in the regulation of cell proliferation and migration, whereas FN isoforms containing the full-length V region (named V120) are ligands to the VLA-4 integrin. To study the changes in the expression of FN isoforms in the anti-Thy-1 nephritis, an acute and self-resolutive model of mesangioproliferative nephritis, we analyzed the FN splicing patterns by means of ribonuclease protection assays. At Day 7 after anti-Thy-1 monoclonal injection, time of the maximal matrix expansion and glomerular hypercellularity, EIIIA+, EIIIB-, and V120 FN mRNA isoforms were increased. In accordance with the mRNA studies, FN proteins, including the EIIIA and V120 regions, increased in the mesangium of nephritic rats, as assayed by immunohistochemistry. Coinciding with the EIIIA and V120 isoforms up-regulation, there was an increase in mesangial cell proliferation and in the number of VLA-4+ infiltrating cells. At Day 14, in parallel with a remission of the above-mentioned changes, there was a decline in the mRNA and protein FN isoforms increased in the previous phase. The marked and reversible changes in the pattern of FN isoforms and their temporal association with other indicators of glomerular injury suggest that certain FN isotypes are important and coordinated components of the mechanisms attempting to reverse glomerular damage.