Changes in the level of glutathione (GSH), the turnover rate, and gamma-glutamyltransferase (GGT) activity were examined in newborn, weanling, and adult male Wistar rats, the objective being to elucidate the mechanisms which control the hepatic GSH level during maturation as well as under conditions of different degrees of protein ingestion. The hepatic GGT activity in the newborn rats was high at birth, decreased within a few days to 1 to 2% of the initial level, and remained unchanged thereafter, when these rats were fed a normal diet after 3 weeks of age. In contrast, the hepatic GSH level increased 3-4-fold while total GGT activity in the kidney increased 6-8-fold. When weanling rats were fed a low protein diet (containing 10% soy protein) for 3 weeks, the hepatic GSH level decreased markedly while the GGT activity increased 5-6-fold. The turnover rate of hepatic GSH also increased, as determined by the use of buthionine sulfoximine, a specific inhibitor of GSH synthesis; a value of 2.1 h was obtained in comparison with 3.5 h for that of rats fed the normal laboratory chow (CRF-1). On the other hand, feeding adult rats on the low protein diet resulted in a marked decrease in hepatic GSH level with no effect on either hepatic or renal GGT activity. These results together with other observations may suggest that GSH translocated out of liver cells in the newborn rats is degraded mainly by these cells, while the tripeptide secreted by hepatocytes of adult rats is metabolized predominantly in extrahepatic tissues, such as the kidney.(ABSTRACT TRUNCATED AT 250 WORDS)