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: These studies were conducted to examine the differential response to a drug challenge under varied experimental test conditions routinely employed to study drug - induced behavioral and neurophysiological responses in rodents .
### Output Text:
These studies were conducted to examine the differential response to a drug challenge under varied experimental test conditions routinely employed to study drug - induced behavioral and neurophysiological responses in rodents . 