Hormones and Homeostatic Mechanisms 169 



Older animals, however, excrete nearly half the water ad- 

 ministered and thus excrete body water via the kidneys. 

 Differences in sodium excretion are also apparent. Thirty- 

 three-day-old animals excrete three times as much body 

 sodium as younger rats. The difference between both age 

 groups studied disappears completely, or becomes much 

 smaller, if 2 • 5 ml. water/100 g. body weight is put into their 

 stomachs two and a half hours before the actual water load. 

 In that case more urine is excreted by the younger animals 

 and losses are reduced in the older age group. Sodium losses 

 are also decreased in the older age group to the same level as 

 in 23-day-old animals. No significant changes in potassium 

 excretion were observed. 



Differences between the two age groups are thus not con- 

 stant. For this reason we assume that the difference is not 

 due only to changes in renal function but that regulatory 

 mechanisms are also concerned. 



The effect of vasopressin was studied in animals receiving 

 one water load and in prehydrated rats. The elimination of 

 the water load was studied according to the method of Falk 

 (1955). In addition the effect on total water loss three hours 

 after the first micturition was studied. This procedure was 

 similar to that of Heller (1952) who determined total renal 

 excretion of a water load 145 minutes after administration of 

 the hormone and the water load. 



After 10 or 25 m-u. vasopressin/100 g. body weight, no 

 significant differences between the two age groups could be 

 observed during water diuresis. This is in agreement with 

 Falk (1955). Yet 23-day-old animals react differently to 

 vasopressin than 33-day-old rats. This difference can be 

 seen in Table I. After a single water load vasopressin (the 

 table shows the results with 25 m-u./lOO g. body weight) 

 increases renal water losses in the younger animals, while in 

 the older group total renal water losses are reduced. The 

 sodium loss in older animals treated with vasopressin becomes 

 greater after prehydration only. In younger animals the 

 elimination of potassium is significantly greater than in the 



