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 μg/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.
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