The relationship between intracellular glutathione (GSH) content and tumor growth remains an important area of ​​research, and RAS genes are among the most mutated proto-oncogenes in cancer. Using optical microscopy and flow cytometry techniques, we have examined the effect of the glutathione (GSH) synthesis inhibitor buthionine sulfoximine (BSO) on cell growth and death in mouse embryonic and RAS-transformed 3T3 fibroblasts. Regarding 3T3 cells and in normal culture conditions, we observe that RAS cells reach higher cell densities as they gradually approach quiescence, are smaller in size, exhibit changes in morphology and distribution (a marked reticular structure with abundant cells at the nodes), and are less attached to the substrate. Our data show that 48 and 72 hours after plating the 3T3 cells, there is: (i) a decrease (directly proportional to the concentration of BSO (25, 50 and 100 µM) in the number of adherent cells and (ii) also cell death with 100 µM of BSO. However, in RAS cells, there is no: (i) decrease in growth at 48 hours, only at 72 hours and with 50 and 100 µM of BSO, or (ii) cell death for any condition. Furthermore, after 48 and 72 hours of incubation with BSO, RAS cells reach cell densities higher than 3T3 cells. And similar results are obtained whether BSO is incubated when the cells are plated or 6 hours later. Finally, preliminary experiments also suggest a shift in the localization of the active growth phase and GSH peak for RAS cells compared to 3T3 cells. In conclusion, 3T3 cells transformed by the RAS proto-oncogene become more resistant to cell death and their growth is less inhibited by BSO. Our detailed analysis of growth under conditions of BSO-induced GSH synthesis inhibition suggests a central role for GSH metabolism and cellular redox homeostasis in RAS protein-mediated tumor development.