ASSAY OF CORTICAL HORMONE 259 



slight loss of weight suffered by rats receiving no therapy. 

 The other curves show the gain in weight of rats receiving 

 various doses of the same charcoal-hormone preparation for 

 seven days. As the amount of hormone administered is 

 increased, the growth curve approximates more and more that 

 of a group of animals subjected to a "control" operation in 

 which the skin and muscle incisions were made without inter- 

 fering with the adrenals. 



As seen in Figure 15, it is easier to determine the amount 

 of hormone necessary for allowing a daily increment of about 

 1.5 grams in weight than to determine the amount necessary 

 for permitting the same rate of growth as is observed in 

 normal animals. Hence, rather than determine the latter 

 quantity, which may be defined as a rat unit, it is easier to 

 determine the minimum quantity of hormone which allows 

 a gain in weight of 1.5 grams per rat per day. This quantity, 

 as may be seen from Figure 15, is about one-half of a rat unit. 



In assaying a given preparation, the number of groups of 

 animals to be used will obviously depend upon the accuracy 

 desired. For an accurate assay of a large batch of available 

 hormone, several groups of animals are maintained on various 

 amounts of hormone as exemplified in Figure 15. Where one 

 is only interested in obtaining an approximately quantitative 

 result, a single group of animals suffices and the dose may be 

 altered from day to day until one approximates the dosage 

 which results in a daily gain in weight of about 1.5 grams per rat. 



After the expiration of the period of treatment (7 to 10 

 days) the animals are maintained on the same diet without 

 addition of the hormone and their weight curves followed, as 

 shown in Figure 15. The cessation of growth after stopping 

 treatment and the subsequent death of the animals in the 

 course of five to ten days is an indication of the completeness of 

 the extirpation. If the adrenalectomy is incomplete the ani- 

 mals continue to grow and live for long periods. One thus has 

 a check upon the completeness of the original operation and 



