The present studies demonstrate, for the first time, that the binding of activator protein-1 (AP-1)-DNA in rat uterus and the estrogen-sensitive areas of the hypothalamus, as measured by electrophoretic mobility shift assay, is increased 2 h after intraperitoneal injection of [D-Met(2),Pro(5)]enkephalinamide. The effect was prevented by the opiate antagonist naltrexone given 30 min before the administration of [D-Met(2),Pro(5)]enkephalinamide, suggesting the involvement of opioid peptide receptors in the observed effects. The present findings support the role of opioid peptides in the regulation of transcription in estrogen-sensitive cells.