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: Conscious SHR were treated for 4 days with prazosin at 2 mg kg - 1 day - 1 i . p . for chronic alpha 1 - adrenoceptor blockade .
### Output Text:
Conscious SHR were treated for 4 days with <span class="chemical">prazosin</span> at 2 mg kg - 1 day - 1 i . p . for chronic alpha 1 - adrenoceptor blockade . 