It has been previously shown that rat glomerular mesangial cells synthesized increased amounts of fibronectin, laminin, and type IV collagen when grown in medium containing 30 mM glucose. High glucose exerted its effect at the mRNA level since transcripts for all three extracellular matrix (ECM) proteins were similarly elevated. High glucose appeared to exert its effect on ECM mRNA levels through protein kinase C activation. Using quantitative reverse transcription (RT) PCR, we now report that mRNA levels for c-fos and c-jun were increased approximately twofold after treatment with high glucose. The fos levels were elevated 15 minutes after addition of high glucose and were maintained elevated through 30 minutes; by one hour mRNA levels for fos returned to control levels. c-jun, on the other hand, was increased at two hours and remained elevated at 24 and 48 hours. Fibronectin mRNA levels were increased three- to fourfold at 24 and 48 hours. Immunofluorescence studies with polyclonal antibodies to c-fos and c-jun revealed that high glucose treatment for four hours increased nuclear staining intensity two- to threefold for both proteins. Nuclear staining for fos returned to control levels by 24 hours while staining for jun remained elevated. These determinations were made on images obtained on a confocal laser scanning microscope. Thus, high glucose may effect gene expression of ECM proteins by elevating the transcription factors c-fos and c-jun which complex with one another to form activator protein 1 (AP-1).