The spatial and temporal expression patterns of cytokeratins, vimentin, epithelial growth factor (EGF) and transforming growth factor alpha (TGF-alpha), were investigated in the 5-9-week old human mesonephros and metanephros. Vimentin was found in all mesonephric structures, while cytokeratins were seen only in the mesonephric tubules. EGF and TGF-alpha were detected early in all mesonephric structures, and immunoreactivity to both factors decreased in later stages. In the 5-6-week metanephros, vimentin immunoreactivity was found in all structures and later increased in the collecting system and interstitium. In the 5th week, cytokeratins 8 and 19 appeared in the ureteric bud and ampullae, and later showed increasing immunoreactivity in the collecting system and nephrons. The coexpression of intermediate filament proteins in metanephric development is a temporary feature and might be associated with mesenchymal to epithelial transformation of developing nephrons. In adult kidneys, such coexpression is associated with fibrosis or carcinomatous changes. At early stages, immunoreactivity to EGF and TGF-alpha was detected in all metanephric structures and from the 7th week onward, it decreased in differentiating nephrons. EGF and TGF-alpha patterns of appearance indicate their role in induction, proliferation and growth of metanephric structures. Disturbances in that pattern might cause reduction in kidney growth.