Abstract: The ultrastructure of the pars anterior (adenohypophysis), treated with diazepam (2.5 mg/Kg) has been studied by thin sections and electron microscopy. The structure of Tirotrophs, Corticotrophs, Gonadotrophs, and Mamotrophs cells were described. Ultrastructural modifications were not observed. It is concluded that the cronic treatment with diazepam did not produce any visible ultrastructural effects in the secretory cells of the adenohypophysis; maybe because the diazepam has a short life time and due to the small effect of the diazepam metabolites in this species.