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Corticosteroid secretion and excretion by the Australian monotreme Tachyglossus aculeatus was investigated by assay of peripheral blood, adrenal venous blood, urine and faeces.Free corticosteroids could be demonstrated in only one out of five samples of peripheral whole blood, in which trace amounts of cortisol and corticosterone were detected at concentrations of the order of 0\m=.\1\p=n-\0\m=.\2 \g=m\g./100 ml.Quantitatively, the major corticosteroid detected in adrenal venous blood of three males and one female was identified as corticosterone, at concentrations of 8\p=n-\25 \g=m\g./100 ml. whole blood. Lesser amounts of cortisol were detected in samples of adrenal venous blood from two males and the female, the ratios 'F:B' being 0\m=.\25and 0\m=.\40in the males, and 0\m=.\13in the female. The highest rate of corticosterone secretion was only 2\m=.\6 \g=m\g./100mg. adrenal/hr. or 1\m=.\17 \g=m\g./kg.body weight/hr. No free or conjugated corticosteroids were detected in either urine or faeces. Zimmermann chromogens were found in both urine and faeces, and total daily excretion varied unpredictably from undetectable levels to a maximum of only 25 \g=m\g./kg.body weight/day.

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Adrenal venous and peripheral blood was collected from five adult and two immature kangaroos, anaesthetized with pentobarbitone.Seven C-21 \ g=D\ 4, 3-oxo steroids were identified in extracts of adrenal venous blood from all animals. The major component\p=m-\72\p=n-\92% of detected corticosteroids\p=m-\was cortisol. The remainder were 11 -deoxy and 21 -deoxy cortisol, corticosterone, aldosterone, 11\g=b\-OH and 17\g=a\-OH progesterone. In four out of six experiments, i.v. corticotrophin caused variable degrees of increase in corticosteroid secretion. Maximum cortisol secretion rates in all the adults and the immature male were in the range 29\p=n-\70\g=m\g./hr./100mg. adrenal, or 28\p=n-\58 \ g=m\ g. / hr. / kg. body weight and 98 \g=m\g./hr./100mg. or 114 \g=m\g./hr./kg. in the immature female.Average aldosterone secretion rate in the adults was 0\m=.\27 \g=m\g./hr./100mg. adrenal, or 0\m=.\18\g=m\g./hr./kg.body weight. It was significantly higher in the immature male. However, the Na status of the animals was not known.Cortisol was the major corticosteroid in peripheral blood plasma at concentrations of 2\ m=. \ 0\ p=n-\ 8\ m=. \ 1 \g=m\g./100 ml.The unusual secretion of 21-deoxy cortisol suggests there may be differences in some aspects of adrenocortical biosynthesis, compared with eutherians.