Ovariectomy caused a significant increase of immunoreactive dynorphin-like material (IR-DYAN) in the anterior pituitary lobe of intact as well as of medial basal hypothalamus-lesioned rats. No change of IR-DYAN was observed in the neurointermediate lobe of the gland or in the hypothalamus. Estradiol benzoate reversed the increase of anterior pituitary IR-DYAN induced by ovariectomy and caused a reduction in sham-ovariectomized rats.