Summary Rat foetuses were subjected to hypophysectomy by decapitation at 181/2 days of development. At 211/2 days the experimental foetuses showed a hypertrophy of the thy-mus and a decrease in adrenal size compared with the litter mate controls When depot ACTH was injected into the foetuses at time of decapitation, this hypertrophy of the thy-mus was inhibited, and the experimental foetuses showed a hypoplasia of the thymus, and a hypertrophy of the adrenals. These results are interpreted as indicating that the foetal pituitary acting through the foetal adrenals is part of a homeostatic mechanism providing an inhibitory regulatory control on thymus growth. These experiments also suggest that the thymus hypertrophy found in human anencephalic foetuses is secondary to the hypoplasia of the foetal adrenals found in this condition.