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: Rats that received ketanserin and alfentanil exhibited less rearing and exploratory behavior at the end of the 60 - min recording period than did animals that received ketanserin alone .
### Output Text:
Rats that received <span class="chemical">ketanserin</span> and <span class="chemical">alfentanil</span> exhibited less rearing and exploratory behavior at the end of the 60 - min recording period than did animals that received <span class="chemical">ketanserin</span> alone . 