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: In sham - operated rats , rilmenidine or alpha - methyldopa elicited similar hypotension that lasted at least 5 h and was associated with reductions in standard deviation of mean arterial pressure .
### Output Text:
In sham - operated rats , <span class="chemical">rilmenidine</span> or <span class="chemical">alpha - methyldopa</span> elicited similar hypotension that lasted at least 5 h and was associated with reductions in standard deviation of mean arterial pressure . 