The adenohypophyseal cells of the pregnant female Myotis myotis present as very active elements organized in anastomosing cords in which several secretory cell types appear to intermingle. The prolactin granules are of variable shape, electron density and diameter (180 to 510 nm). The granules of the gonadotrophic cells are columnar or cuboidal and are of medium electron density; the majority are smaller than the prolactin granules. The adenohypophysis of pregnant females kept under experimental conditions shows disorganization of the cell cords and degranulation of the prolactin and gonadotrophic cells, which display greatly increased development of the rough endoplasmic reticulum and Golgi apparatus, a large number of lysosomes and a large amount of degraded material.