ASSAY OF CORTICAL HORMONE 251 



to be entirely impracticable. The same objections as have 

 been outlined above are applicable to assay methods based on 

 the changes occurring in other blood constituents (e.g., the 

 sodium chloride) of adrenalectomized animals. In general, 

 methods involving the use of adult adrenalectomized animals 

 are much less satisfactory than those based on the use of young 

 immature animals. Extraneous factors, such as the adminis- 

 tration of excessive amounts of salt in the diet, will give 

 deceptive results in adult animals. In young animals, on the 

 other hand, such factors are less apt to vitiate the results, as 

 shall be demonstrated in the next section. Many erroneous 

 conclusions found in the recent literature are to be attributed to 

 a failure to appreciate the ease with which an assay method 

 may be vitiated by the factors outlined above. 



THE RAT METHOD OF ASSAY 



The crucial and direct test of the potency of an adrenal extract 

 is its ability to maintain an adrenalectomized animal in perfect 

 health. To apply such a test to an adult cat or dog would 

 require, as we have seen, a period of observation of at least a 

 month on several animals and involve the use of large quanti- 

 ties of extract The use of the month-old-rat, according to 

 the technique described in the present section, affords a simple 

 and accurate method of assay which involves the use of 

 minimal amounts of extract, can be performed with a modicum 

 of effort, and gives results which are reproducible with a varia- 

 tion of less than 15 per cent. The method depends upon 

 determining the growth curve of month-old adrenalectomized 

 rats for a period of one week during which they are main- 

 tained on the cortical hormone. Month-old rats are used 

 because complete adrenalectomy is relatively easily performed 

 on these animals, their normal survival period is short (less 

 than a week), and their normal rapid growth ceases following 

 adrenalectomy. Moreover, the results obtained on immature 

 animals are less apt to be vitiated by extraneous factors, such 



