1. The major corticosteroids aldosterone and cortisol (corticosterone in rodents) are secreted from the adrenal cortex under the regulation of the renin-angiotensin system and the hypothalamic-pituitary-adrenal axis. 2. In addition to their accepted roles in such processes as blood pressure regulation, glycogenesis, hepatic glyconeogenesis and immunosuppression, the corticosteroids have been implicated in the development of cardiac fibrosis, modulation of hippocampal neuron excitability, memory formation and neurodegeneration. 3. The advent of sensitive molecular biological techniques has produced a wealth of evidence to support the existence of extra-adrenal corticosteroidogenic systems. Most attention has been paid to the cardiovascular system and the central nervous system, where the full array of enzymes required for the de novo synthesis of corticosteroids from cholesterol has been identified. 4. Although the evidence for local corticosteroid production is strong, the quantities of steroid would be small compared with adrenal production. Therefore, it is still a matter of debate as to whether extra-adrenal corticosteroids are of any physiological significance. This will depend on factors such as local concentration, proximity to target cells and, possibly, to tissue-specific control mechanisms.