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Therefore , in the present experiment , the effects of clonidine administration ( 0 . 5 mg / kg ) on cardiac rate and ultrasound production were examined in 2 - , 8 - , 15 - , and 20 - day - old rats .
 clonidine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 clonidine