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: Intraperitoneal ketanserin administration at doses of 0 . 63 and 2 . 5 mg / kg prevented the alfentanil - induced increase in electromyographic activity compared with animals pretreated with saline .
### Output Text:
Intraperitoneal <span class="chemical">ketanserin</span> administration at doses of 0 . 63 and 2 . 5 mg / kg prevented the <span class="chemical">alfentanil</span> - induced increase in electromyographic activity compared with animals pretreated with saline . 