We have analyzed the domain structure of the mouse glucocorticoid receptor by expression of in vitro mutated receptor in COS-7 cells. The receptor consists of a core domain rich in Cys, Lys, and Arg amino acids which can bind specific DNA sequences (glucocorticoid response elements) and activate transcription. The activity of this centrally located domain is modulated by the activity of the other two domains. The N-terminal domain of the receptor plays a role in decreasing nonspecific DNA binding and may therefore improve the ability of the protein to discriminate between specific and nonspecific DNA binding sites. This activity maps to a small, highly acidic region of the N-terminal domain. The C-terminal domain of the receptor contains the glucocorticoid binding site and in addition represses the transcriptional activity of the receptor in the absence of hormone. Hormone binding relieves the repression allowing transcription activation. The C-terminal domain contains a short sequence conserved among steroid receptors; its deletion yields a receptor that activates transcription in the absence of hormone.