An obese (ob) gene product, expressed specifically in adipose tissues, regulates energy balance. Here we report that adipocytes in adipose tissue actually express ob mRNA and that a synthetic glucocorticoid (dexamethasone) regulates expression of the ob gene. Addition of 100 nM dexamethasone to isolated rat adipocytes rapidly induced a 4-8 fold increase in ob mRNA. This increase in ob mRNA level was apparent within 1 h, and reached a maximum at about 7 h after stimulation. The dexamethasone-stimulated increase of ob mRNA was only partially blocked by protein synthesis inhibitors cycloheximide (20 μg/ml) or anisomycin (200 μM). This suggests that new protein synthesis is not necessarily required for the observed dexamethasone-stimulated increase in ob mRNA.