Gamma glutamyltransferase (GGT) is a membrane-bound enzyme that is involved in glutathione metabolism and aminoacids uptake. GGT activity is stimulated by a number of hormones and pharmacological agents in certain animal tissues. In rat liver, adrenalectomy causes a 2-fold decrease in GGT activity and hydrocortisone treatment of adrenalectomized animals specifically stimulates this enzyme activity. In a highly differentiated hepatoma cell line, Fao, GGT activity is similar to rat liver and is under glucocorticoids control. These hormones specifically stimulate GGT activity (2- to 3-fold). Translation and transcription inhibitors prevent the hormonal effect. The stimulation of GGT activity is therefore probably due to an increase in GGT mRNA synthesis. The results reported suggest that the Fao cell line is a very convenient system for the study of the molecular mechanisms of both the glucocorticoid effects on differentiated cells as well as the modulation of membrane-bound enzymes biosynthesis.