The development and function of the primate adrenal cortex are characterized by rapid growth, high steroidogenic activity, and a particular morphological appearance. The fetal adrenal glands grow rapidly and exponentially and at term are similar in weight to adult adrenals. From birth to 1 year their mass is reduced as they undergo a process of differentiation. Growth then remains slow until age 7 years. Thereafter, growth accelerates and the adrenals reach adult weight by the end of puberty. In the first trimester of gestation, fetal adrenal growth is thought to be independent of adrenocorticotropic hormone (ACTH), but after 15 weeks, ACTH is absolutely required for normal morphological and functional development. Other factors of fetal and/or placental origin, acting independently of or in conjunction with ACTH, are also required. Basic fibroblast growth factor, epidermal growth factor/transforming growth factor beta, and insulin-like growth factor (IGF)-I and -II, all acting in an autocrine and/or paracrine fashion, have been postulated to stimulate fetal adrenal cell proliferation. Corticotropin-releasing hormone may also play an important role in primate fetal adrenal function, primarily at the end of gestation. Finally, the estrogens are also important in the development of the pituitary-adrenal axis in primates.