Buthionine sulphoximine (BSO) an inhibitor of glutathione (GSH) biosynthesis, was administered to mice in single and repeated doses of 0.5, 1 and 5 mmol kg-1 (i.p.). The resultant pattern of GSH depletion was studied in liver, kidney, skeletal muscle and three types of murine tumor. Liver and kidney exhibited a rapid depletion to GSH levels of ca. 20% of controls after single doses of 1-5 mmol kg-1 BSO. Muscle was depleted to a similar level, but at a slower rate after a single dose. All three tumors required repeated administration of BSO over several days to obtain a similar degree of depletion to that shown in the other tissues.