The corticosterone concentration was assayed by the method of MURPHY (1967), in the plasma of the maternal and foetal rat submitted to different experimental conditions, after an isooctane washing to remove the progesterone. In foetuses, decapitated on day 18, from adrenalectomized mothers, the plasma corticosterone concentration was much lower than in the controls, on day 21. An injection of synthetic 1-24 corticotrophin at the moment of decapitation suppressed this effect. When the mother was not adrenalectomized, the plasma corticosterone values in the decapitated foetuses were near the control levels. This last finding confirms the transfer of corticosteroids through the placenta from the mother to the foetus. An injection of 1-24 corticotrophin to normal foetuses, on day 18, produces a large increase in plasma corticosterone in comparison with the control foetuses, on day 21.