Glucocorticoid agonists, i.e. dexamethasone or triamcinolone acetonide, rapidly induce expression of CCAAT/enhancer-binding protein (C/EBP) delta and repress expression of C/EBP alpha in fully differentiated 3T3-L1 adipocytes. Within 30 min of glucocorticoid treatment, the cellular level of C/EBP delta rises dramatically, increasing > 100-fold within 6 h. Concurrently, the level of C/EBP alpha decreases, reaching a minimum within 4 h. The dexamethasone concentration dependence and steroid specificity of these responses suggest that both processes are mediated by the glucocorticoid receptor. The reciprocal effects of dexamethasone on the steady-state levels of C/EBP alpha and C/EBP delta can be accounted for kinetically and quantitatively by changes in their mRNA levels and by the transcription rates of their respective genes. The glucocorticoid-induced changes in expression of the C/EBP isoforms are correlated with the transcriptional activation of the SCD1 gene, an adipocyte gene known to be transactivated by C/EBP isoforms. Glucocorticoids also regulate expression of the C/EBP isoforms in vivo. Within 4 h of administration of dexamethasone or triamcinolone acetonide to adult rats, expression of C/EBP delta is induced in white adipose tissue while expression of C/EBP alpha is repressed. Like the response in 3T3-L1 adipocytes, the effects of dexamethasone on C/EBP alpha in white adipose tissue are rapid and transient.