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: Thus , we now assessed the depressive - like behaviour in mice at early and long - term periods following a single high METH dose ( 30 mg / kg , i . p . ) .
### Output Text:
Thus , we now assessed the depressive - like behaviour in mice at early and long - term periods following a single high <span class="chemical">METH</span> dose ( 30 mg / kg , i . p . ) . 