In order to test the possible role of estradiol in the response of the LH-adenohy-pophyseal cells to the administration of met-enkephalin in the albino male rat, an immunocytochemical (peroxidase-antiperoxidase) morphometric (cellular and nuclear areas and numerical density) and radioimmunoassay (LH serum levels at the moment of sacrifice) study was carried out. The intraventricular administration of met-enkephalin (150 µg in 25 µl of distilled water) does not produce any changes in the parameters considered. However, when the animals were pretreated with estradiol (chronically, 15 days), met-enkephalin produced a significant decrease in all the parameters considere