The purpose of this study is to determine the phenotype of extracellular matrix molecules in normal adult mouse glomeruli. We used the increased sensitivity afforded by the polymerase-chain reaction to assess type I and type IV collagen mRNA in freshly microdissected normal adult mouse glomeruli. RT-PCR reactions for mRNA encoding these components were also performed using mesangial cell lines previously isolated from the same strain of mice. Type IV collagen mRNA was easily detectable in normal adult mouse glomeruli as well as in the cell lines. On the other hand, type I collagen mRNA was not detected in the glomeruli, despite increasing the number of PCR cycles from 25-45 (roughly a 1000 fold increase in sensitivity). Assays using competitive PCR were also developed. Utilizing the same primers, type I collagen mRNA was easily demonstrable in two lines of mouse mesangial cells. These experiments support data in both humans and experimental models which failed to demonstrate type I collagen by immuno-fluorescence microscopy in normal glomeruli, whereas type IV collagen was present in large amounts. The current study provides evidence that the expression of types I and IV collagen in normal glomeruli is regulated at the pretranslational level in vivo.