Mercury is a major component of dental amalgam and is often used in medical disinfectants. However, this metal is very toxic to all organisms. In particular mercury vapor (Hg*) and methylmercury are potent neurotoxins; however, neither the mechanism by which methylmercury enters the brain nor the mechanism by which these mercurials cause neurotoxicity is known. The long term goal of this research is to understand the interaction and toxicity of mercurials with the cells that constitute the blood-brain barrier, brain capillary endothelial cells. Glutathione is a tripeptide which plays major role in the detoxification of mercury and many electrophilic molecules. The enzyme responsible for facilitating the metabolism of glutathione and glutathione S-conjugates gamma-glutamyl transferase (gamma-GT). Gamma-GT is an ectoprotein located on the luminal surface of brain capillary endothelial cells and functions as a biological marker for these cells. Our recent studies examined changes in gamma-GT activity during culture of brain endothelial cells and the effect of some growth factors on gamma-GT expression. Methods were established to further characterize a primary cell culture system of bovine brain capillary endothelial cells. The cells were isolated, the incubated in 95% air and 5% CO2. A method was developed to measure the levels of cellular gamma-GT per weight of DNA. At confluency the cells were dosed with media containing either tumor necrosis factor (TNF), endothelial growth supplement (EGS), or fibroblast growth factor (FGF). Preliminary results suggest that the units of gamma-GT per ug of DNA were greater in cells treated with TNF, compared to cells treated with FGF or EGS. Future studies will examine the role of gamma-GT in mercury uptake and toxicity in these cells. KEY WORDS: Mercury toxicity, blood brain barrier, neurotoxins, cell culture, gamma glutamyl transferase, glutathione