In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Sodium thiopenthal was administered intraperitoneally into male rats pre - treated with melatonin ( 0 . 05 , 0 . 5 , 5 and 50 mg / kg ) .
### Output Text:
<span class="chemical">Sodium thiopenthal</span> was administered intraperitoneally into male rats pre - treated with <span class="chemical">melatonin</span> ( 0 . 05 , 0 . 5 , 5 and 50 mg / kg ) . 