The effect of steroids on the growth of AtT-20 mouse pituitary tumor cells was investigated. It was found that under certain conditions glucocorticoids inhibit growth. Specificity studies indicated that only glucocorticoids caused this effect. Biopotency studies indicated that dexamethasone had its mid-maximal effect around 5 nM. At least part of this inhibition was caused by a decrease in the cell's ability to synthesize DNA; glucocorticoids inhibit the rate at which tritiated thymidine is incorporated into TCA-precipitable material. The magnitude of the growth-inhibiting effect depended upon the prior culture history of the cell. The effect was least on cells derived from exponentially growing cultures and most effective on cells derived from cultures that were approaching their density limit. It is concluded that glucocorticoids inhibit AtT-20 cell growth by interfering with their ability to transform from confluent cultures to ones that grow exponentially. The significance of this finding is that now two mechanisms must be considered when investigating the pathways through which glucocorticoids decrease the ACTH production of the AtT-20 cell.