The glutathione level and the factors affecting this level were investigated in fetal rat brain cells in a primary culture. Early in the culture, the glutathione level of the brain cells decreased, but after 5 h it began to increase VSports手机版. This increase was not observed in a cystine-free medium and was prevented by excess glutamate. Cystine was taken up in freshly isolated brain cell suspensions, and its rate increased during the culture. The cystine uptake was mediated by a Na(+)-independent, glutamate-sensitive route previously found in various types of cells and designated as system X-c. The uptake of cystine is a crucial factor in maintaining the glutathione level of the cells under culture, because it provides cysteine for the cells for glutathione synthesis. Cysteine was undetectable in the medium before the culture, but it appeared, though at a very low level, when the brain cells were cultured there. The source of this cysteine was the cystine in the medium. Presumably the decrease in the glutathione level of the cells in the early stage of the culture resulted from the fact that the medium did not contain cysteine. The enhancement of the cystine uptake during culture may constitute a protective mechanism against the oxidative stress to which the cultured cells are exposed. Regulation of the glutathione level in fetal brain cells in vivo by the transport of cystine and cysteine is discussed. .