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: Rats were pretreated with saline or nicotine ( 1 . 5 mg / kg per day ) by subcutaneously implanting each animal with an Alzet osmotic mini - pump which continuously released saline or nicotine for 1 , 5 and 14 days .
### Output Text:
Rats were pretreated with saline or <span class="chemical">nicotine</span> ( 1 . 5 mg / kg per day ) by subcutaneously implanting each animal with an Alzet osmotic mini - pump which continuously released saline or <span class="chemical">nicotine</span> for 1 , 5 and 14 days . 