Plasma aldosterone, cortisol and corticosterone concentrations in the new-born guinea-pig.

1. Plasma aldosterone concentrations have been measured by radio-immunoassay in the new-born and adult guinea-pig and related to plasma cortisol and corticosterone concentrations.2. Plasma aldosterone concentrations were high in the hours immediately after birth with peak values of 552 pg/ml. at between 6 and 24 hr of age. Adult male plasma concentrations were 72 pg/ml. in plasma taken by cardiac puncture and 126 pg/ml. when measured in blood taken from an indwelling arterial catheter.3. The major plasma glucocorticoid was cortisol and earlier reports of values above 1 mug/ml. were confirmed by more exacting column chromatography with Sephadex LH 20.4. There was no constant relation between aldosterone and glucocorticoid concentrations in the animals studied, either new-born or adult.5. Plasma cortisol: corticosterone ratios were not constant but varied considerably. In the adult a sevenfold rise in plasma cortisol concentration occurred under certain conditions without any change in plasma corticosterone concentrations.