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: Head - up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of prazosin ( 0 . 1 mg kg - 1 i . p . ) .
### Output Text:
Head - up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of <span class="chemical">prazosin</span> ( 0 . 1 mg kg - 1 i . p . ) . 