Hormones and Homeostatic Mechanisms 177 



of the sudanophil layer increases in both age groups but in 

 the younger group the size of the whole cortex is smaller. 

 It is difficult to interpret these changes. It is certain, however, 

 that according to morphological criteria the adrenals of the 

 23-day-old animal react differently from those of the animal 

 aged 33 days. This would indicate that changes in the reac- 

 tivity of infant rats to a water load at the end of the natural 

 period of weaning, and to corticoids, are also conditioned by a 

 different reactivity of the adrenals and the adrenopituitary 

 system. 



This hypothesis is further supported by results from ex- 

 periments in which the effect of ACTH and a combination of 

 ACTH and cortisone (0-25 mg./lOO g.) was studied on the 

 elimination of water, sodium and potassium after a water 

 load. Results are shown in Fig. 7. As has already been shown, 

 cortisone prevents retention of a water load in 23-day-old 

 animals and considerably increases renal water losses. ACTH 

 is without effect. After simultaneous administration of ACTH 

 and cortisone, water losses decrease in comparison to losses 

 after cortisone only. In 33 -day-old rats results are less 

 evident because of the large scatter. ACTH itself causes an 

 increase in sodium excretion in 23-day-old animals but in 

 combination with cortisone it is without effect on sodium 

 elimination and thus removes the latter's natriuretic effect. 

 This effect is probably due to the lower renal water losses. 

 In 33-day-old animals ACTH decreases sodium losses just as 

 do cortisone and cortisone combined with ACTH. The same 

 holds good for ACTH when combined with cortexone. ACTH 

 prevents atrophy of the adrenals after cortisone in infant 

 rats aged 23 days and also prevents the effect of cortisone on 

 sodium and water elimination. This is not the case in older 

 animals. This is in agreement with the histological picture 

 and with the differences between 23 and 33-day-old animals. 



We have thus been able to show that there is a time 

 correlation between changes in homeostatic mechanisms 

 regulating the intake of water and electrolytes appearing in 

 infant rats at the time of natural weaning, and adrenal 



