Thy-1 expression may influence myofibroblast development. Through the epithelial-mesenchymal transition (EMT), injured renal epithelial cells undergo regression to the metanephric mesenchymal phenotype and then acquire a myofibroblastic nature (expressing α-smooth muscle actin; α-SMA). Because the metanephric blastema differentiates into mesenchymal and renal epithelial cells, we investigated Thy-1 immunoexpression during nephrogenesis in F344 rats in correlation with vimentin and α-SMA expressions. Kidney samples were obtained from fetuses on gestation days 18 and 21, neonates on days 1-18 and adults at 6 weeks of age. Mesangial cells in S-shaped bodies and immature and mature glomeruli continuously expressed both Thy-1 and α-SMA during early nephrogenesis (fetuses and neonates on days 1-9). During early nephrogenesis, loosely-arranged blastemal cell-derived mesenchymal cells in the cortex and medulla also exhibited Thy-1 and α-SMA, although the α-SMA expression was weaker than that of Thy-1. Vimentin expression coincided with that of Thy-1. These findings indicate that the derivation of α-SMA-expressing myofibroblastic cells may be related to mesangial or blastemal cells expressing both Thy-1 and α-SMA. Interestingly, there was a difference in Thy-1 expression between cortical and medullary tubulointerstitial cells from late nephrogenesis (neonates on days 12-18) and those from adults in that the cortical cells reacted faintly or negatively to Thy-1, whereas the medullary cells reacted strongly to Thy-1; additionally, bundle-arranged mesenchymal cells that were only observed in the neonates on days 1-12 reacted strongly to α-SMA, but faintly to Thy-1. Blastemal cell-derived mesenchymal cells seem to alter the immunoexpressions of Thy-1 and α-SMA, depending on the conditions which they develop. Thy-1 immunoexpression would be useful for investigation of reverse embryogenesis, which might occur in fibrotic kidneys.