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: The locomotor activity was decreased from corresponding controls in all strains studied , except for the ICR mice , during an overnight drug - free period following the fourth amantadine treatment .
### Output Text:
The locomotor activity was decreased from corresponding controls in all strains studied , except for the ICR mice , during an overnight drug - free period following the fourth <span class="chemical">amantadine</span> treatment . 