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 the present study the role of habituation to the experimental environment on the stimulant effect of nicotine in rats was examined .
### Output Text:
In the present study the role of habituation to the experimental environment on the stimulant effect of <span class="chemical">nicotine</span> in rats was examined . 