166 Ji^i Kre(5ek, et al. 



drink water. Gradually the mechanisms for compensation of 

 thirst and hunger separate. At the end of the fourth week 

 infant animals cease to feed on breast milk and take in food 

 that is normal for adult animals. 



We studied the active intake of water, electrolyte solutions 

 and milk in infant rats using the method of free choice as 

 known especially from the work of Richter (1936), Young 

 (1949), and Young and Chaphn (1949). We observed that in 

 infant rats weaned at the beginning of the third week of 

 postnatal life there is a significant change in the regulation 

 of water, electrolyte and milk intake at the end of the fourth 

 week. The regulation of sodium intake in relation to water 

 intake, especially, changes. According to Richter (1936) 

 appetite for individual components of the diet is an important 

 homeostatic mechanism and is determined by the needs of the 

 organism. 



In order to be able to offer a physiological explanation for 

 changes in the regulation of sodium intake it is necessary to 

 throw light on the relation between mechanisms of self- 

 selection and other components of water and electrolyte 

 metabolism that can be studied better and more objectively. 



The adrenals and the posterior lobe of the pituitary are of 

 special significance for the regulation of water and electrolyte 

 metabolism. For this reason we have studied the effects of 

 hormones from these two glands. Up to the present nothing 

 is known of a change in function of the adrenals or in the 

 effect of their hormones at the end of the fourth week of life 

 in the rat. Indirectly one might expect such a change from 

 the fact that the regulation of sodium intake depends on the 

 function of the adrenals (Richter, 1936). There is also no dif- 

 ference in the size of the glands in males or in females during 

 the fourth week. 



More is known about changes in the role played by the 

 posterior lobe of the pituitary during this period. Heller 

 (1952) showed that up to the end of the fourth week of life the 

 rat kidney does not react to vasopressin during a water load 

 in the same way as that of the adult. In addition the ability 



