The human foetal adrenal produces hydrocortisone in barely measurable amounts when cultured alone, but in considerable amounts for 8–10, and occasionally up to 25, days when cultured in combination with human foetal hypophysis (Stark, Gyévai, Szalay & Ács, 1965). This may be regarded as evidence of the ability of the human foetal hypophysis to secrete ACTH and to do so independently of the hypothalamus, provided it can be demonstrated that it is not ACTH accumulated in situ in the foetal hypophysis, and gradually washed out from the explant, that increases the amount of corticosteroids in the culture medium. The present experiments were done on the assumption that—since in a single-cell culture growth by cell division is very rapid—any ACTH present in a monolayer culture of trypsinized hypophysial cells would probably result from productive cellular activity, if the fluid medium was changed daily throughout the life of the culture. Adrenals and