1. Glutathione concentrations in liver and lung fall when food intake or sulphur amino acid intake is inadequate. However, concentrations may be restored during inflammation, despite anorexia, provided that prior sulphur amino acid intake is adequate. 2. We studied the mechanisms of these changes by measuring the effect of sulphur amino acid and protein intake on hepatic glutathione synthesis and gamma-glutamylcysteine synthetase activity, hepatic and lung glutathione concentrations, glutathione reductase and glutathione peroxidase activities in young rats given an inflammatory challenge by intraperitoneal injection of tumour necrosis factor-alpha or endotoxin (lipopolysaccharide). 3. Diets containing 200 g of casein and 8 g of L-cysteine/kg (normal-protein diet), or 80 g of casein and 8 g of L-cysteine, or isonitrogenous amounts of L-methionine or L-alanine (low-protein diets) were fed ad libitum to young Wistar rats for 8 days. Dietary groups were subdivided into three: one subgroup continued feeding ad libitum, a second was given tumour necrosis factor or lipopolysaccharide and killed 24 h thereafter, while the third was pair-fed to the intakes of the second subgroup for 24 h before being killed. 4. Glutathione concentrations in liver and lung were reduced in rats fed the low-protein diet containing alanine, and in all dietary groups when food intake was restricted. The inflammatory challenges restored hepatic glutathione concentrations in all groups but the diet supplemented with alanine, which had an inadequate sulphur amino acid content. In lung, restoration occurred only in animals fed the normal-protein diet. 5. The activity of gamma-glutamylcysteine synthetase, which is rate limiting for glutathione synthesis, was unaffected by dietary or sulphur amino acid intake or by the inflammatory response. Substrate supply may therefore be a major determinant in glutathione synthesis in vivo. 6. Total hepatic glutathione synthesis was affected by food intake, the type and amount of sulphur amino acids in the diet and by inflammation. Total synthesis was 207, 137, 421 and 90 mumol/day for animals fed ad libitum the normal-protein diet, or low-protein diets supplemented with cysteine, methionine or alanine respectively, ad libitum. Pair-feeding resulted in values of 76, 31, 71, and 0 mumol/day respectively. After lipopolysaccharide injection, rates increased to 200, 117, 151 and 56 mumol/day respectively. 8. Reductase and peroxidase activities increased in liver and lung, when low-protein diets which contained supplemental methionine or alanine were consumed ad libitum. A reduction in food intake resulted in enzyme activity changes, which suggested that recycling of glutathione increased in lung and decreased in liver. Injection of tumour necrosis factor reversed this effect. 9. The restoration of glutathione concentrations in liver after an inflammatory challenge is closely associated with an enhanced rate of synthesis and increased recycling. The former is impaired when inadequate sulphur amino acid is consumed before the challenge. In lung, increased recycling of glutathione may help maintain concentrations when food intake is restricted, but not during inflammation.