SUMMARY The work was designed to investigate maternal/foetal adrenocortical interrelationships and, in particular, to answer three questions, namely: (i) what are the factors which control the growth of the foetal adrenal gland; (ii) does the foetal adrenal secrete hormones which have a biological action, similar to adult corticosteroids, on water and salt-electrolyte metabolism; and (iii) is the improved health of the adrenalectomized pregnant rat, compared with that of the adrenalectomized non-pregnant rat, occasioned by the presence of foetuses with their adrenals, or by the secretion of progesterone accompanying pregnancy? Various categories of experimental animals were considered, including the following: normal non-pregnant and pregnant animals; normal and adrenalectomized non-pregnant animals injected with progesterone; pseudopregnant and adrenalectomized pseudopregnant animals; adrenalectomized non-pregnant and pregnant animals and those with the foetuses removed; animals in which ACTH injections were made into adrenalectomized mothers or directly into foetuses; hypophysectomized non-pregnant and pregnant animals; hypophysectomized and adrenalectomized pregnant animals; normal and adrenalectomized mothers with decapitated foetuses. Adult and foetal body and adrenal weights and foetal pituitary weights were recorded, and the adrenals in both examined histologically. The sodium and potassium content of adult and foetal plasma and muscle, and the water content of muscle, were determined. The variations in body weight and of food and water intakes of animals in the different experimental categories were observed. It was concluded that enlargement of the foetal adrenal after adrenalectomy of the mother was due to activity of the foetal pituitary. There was no evidence that either normally or under special circumstances the foetal adrenal secreted hormones which influenced water and salt-electrolyte metabolism. The sodium content of foetal plasma followed fairly closely that of maternal plasma in the various groups. The foetal plasma potassium content (which was about twice that of the adult), the foetal muscle sodium content (which was about eight times that of the adult) and the foetal muscle potassium content (which was similar to that of the mother), did not vary greatly, nor did they seem to be conditioned hormonally. The presence of foetuses in adrenalectomized mothers did not alter the salt-electrolyte changes usually occurring in cortical insufficiency. While progesterone had a beneficial effect on appetite and body weight in adrenalectomized animals, it tended to produce an increased potassium content of muscle but did not change the salt-electrolyte content of plasma, so that the nature of its action is not clear.