The ovine fetal adrenal cortex and pituitary are functional secretory organs by the end of the first third of gestation (term is 142-152 days). By half-way through gestation the zona glomerulosa is mature morphologically, more than 80% of the aldosterone in fetal blood is of fetal adrenal origin, but conventional stimuli, for example, increased plasma K+ or angiotensin II, do not increase aldosterone secretion until near term. The zona fasciculata is immature histologically, relatively unresponsive to ACTH, and contributes less than 10% of the cortisol in fetal blood between 100 and 120 days of gestation. After this time the zona fasciculata cells begin to mature, to respond to ACTH and to produce an increasing proportion of the cortisol in fetal blood. A functional relationship between hypothalamus-pituitary-adrenal cortex matures over the last fifth of gestation. It is hypothesized that cortisol exerts a local effect in maturation of fetal zona fasciculata cells, such that low concentrations of ACTH have increasingly larger effects on growth and secretion of the fasciculata and that the level of negative feedback by cortisol on the hypothalamic-pituitary axis is reset. The analogy is drawn between the changes in gonadotrophin and gonadal hormones which culminates in puberty in man and the changes in ACTH and cortisol which culminate in parturition in sheep.