Plasma and adrenal aldosterone concentrations have been estimated by radioimmunoassay and by gas phase chromatography in the foetus, the newborn and the mother between day 62 of pregnancy and day 8 of post-natal life. The pre-natal period is characterized by a gradual increase in plasma and adrenal aldosterone levels from day 62 up to 67. Birth greatly stimulates the synthesis and the release of aldosterone. In the neonatal period there are two peaks of maximum intensity in the plasma aldosterone concentration, one at 12 h together with a high adrenal aldosterone level, the other on day 5. In the mother the plasma aldosterone concentration is always higher than that of the foetus during the pre-natal period and increases from day 64 of gestation until parturition and remains high during the first 24 h after parturition.