Endocrinology was recognized as a new branch of biological science mainly as a result of events which took place between about 1890 and 1905, but ideas and discoveries dating from antiquity contributed to it also. Experiments supporting the concept of internal secretions by the testicles were described by Aristotle (4th c. B.C.) and by Hunter (18th c.) and Berthold (19th c.). In 1855 Bernard described glucose as an internal secretion of the liver and Addison reported the effects of adrenal disease in man. Adrenalectomy was fatal in animals. Goitre was known in antiquity and cretinism had been described by Paracelsus. Myxoedema was reported by Gull in 1873, and Kocher described cachexia strumipriva in 1883. In 1888 cretinism, myxoedema and cachexia strumipriva were attributed to thyroid insufficiency. In the 1890s Gley found that tetany after thyroidectomy was due to removal of the parathyroids. In 1884 Rehn proposed that toxic goitre was due to thyroid excess. In 1889 Brown-Séquard claimed that injections of testicular extract rejuvenated the elderly, and in 1893 he introduced organotherapy. In 1891 Murray treated myxoedema successfully with thyroid extract. In 1893 Oliver and Schäfer found that an adrenal extract raised the blood pressure, and soon adrenaline was extracted from the adrenal medulla. Adrenocortical deficiency was proposed as the cause of Addison's disease, and in 1896 Osler prepared an extract which relieved one patient. Diabetes mellitus, described in the first century, was usually fatal. Thirst and polyuria followed experimental pancreatectomy, and pancreatic lesions were found in some human diabetics. In the 19th century workers in France and Germany found that diabetes resulted from absence of an internal secretion by the islets of Langerhans and, in 1893, Laguesse described the function of the islets as "endocrine". In 1895 Beatson treated advanced breast cancer successfully by oöphorectomy. In 1895 Schäfer commended study of the internal secretions to physiologists. In 1902 Bayliss and Starling discovered secretin, a chemical messenger secreted by the intestinal mucosa. In 1905 Starling proposed the name "hormone" for this class of internal secretions. By then endocrinology had been launched as a new branch of science. The crucial events which led to the recognition of endocrinology as a new branch of biological science took place between about 1890 and 1905. Many ideas and discoveries dating from antiquity and apparently unrelated at first had, however, contributed to it. Most of the organs and tissues that form the endocrine system were recognized over 100 years ago.(ABSTRACT TRUNCATED AT 400 WORDS)