Publisher Summary The chapter discusses drug-induced changes in the endocrine glands. The endocrine system is traditionally regarded as a system of glands capable of releasing chemical mediators that act on targets distant from the site of production. Findings show that although endocrine organs are usually resistant to direct toxic effects of xenobiotics, they are extremely sensitive to stimulation or inhibition by a variety of trophic or antitrophic substances and by end-organ feedback. A difficulty in the interpretation of drug-induced changes in the endocrine tissue is the distinction between hyperplasia, and benign and malignant neoplasia on morphological grounds. The pituitary gland or hypophysis is composed of an anterior lobe or adenohypophysis, embryonically derived from the hypophyseal recess or Rathke's pouch, and a posterior lobe or neurohypophysis developing from diencephalic neuroectoderm. Weighing the pituitary gland is a time-honored method for the detection of hypertrophy and hyperplasia. Hypertrophy and hyperplasia of the pituitary gland are particularly well studied in the rat, but similar changes seem to occur in other species including humans. Findings show that florid patterns of diffuse hyperplasia are induced by much longer periods of treatment with high doses of estrogenic agents, although these changes are reversible.