Mouse fibroblasts growing in vitro respond to glucocorticoids in a dose-dependent fashion by reduced rates of growth. The inhibition of growth observed in vitro is related to the topical anti-inflammatory action of glucocorticoids and to their capacity to inhibit wound repair. The cells growing in vitro possess a glucocorticoid receptor system that has been studied in some detail using [3H]triamcinolone acetonide as a radiolabeled ligand. The initial binding reaction occurs in the cytosol. The complex is then rapidly taken up in the nucleus of the cell by a temperature-sensitive process. In the nucleus, the complex exists in two forms, one of which is readily extracted by 0.3 M KCl solutions. A small amount of steroid-receptor complex is tightly bound to chromatin. Under normal incubation conditions, there is a constant cycling of steroid-receptor complex, and unbound receptor is generated back into the cytosol from the nucleus with a half-life of about 30 min. Regeneration of unbound receptor does not depend on protein synthesis and is a temperature-sensitive and energy-requiring process. Incubating the steroid-treated cells in the absence of glucose and in the presence of inhibitors such as cyanide or dinitrophenol leads to a loss of cytoplasmic steroid-receptor complexes, and an accumulation of the complex in the nuclear residual form, tightly bound to chromatin. With respect to nuclear effects of steroid treatment, we have found that incubating fibroblasts in vitro with glucocorticoids produces a prompt decrease in the amount of a satellite H1 histone found in these cells.