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The glutamine synthetase system of strain L mouse cells is regulated by at least two natural substances: glutamine and a glucocorticoid hormone. In a chemically defined cell culture system, cortisol or dexamethasone promotes intracellular accumulation of the synthetase by a process which depends upon genetic transcription. The accumulation is most rapid if the hormone is added to a glutamine deficient culture medium. High concentrations of glutamine promote rapid disappearance of the enzyme by a mechanism which depends on unimpaired protein synthesis. Ways in which the two regulatory substances collaborate to influence the synthetase level and other growth-related processes of L cells in culture are discussed. [ABSTRACT FROM AUTHOR]