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 this investigation the behavioural effects of chronic ( two week ) treatment with 5 - FU and ( three weeks ) with Fluoxetine either separately or in combination with 5 - FU were tested on adult Lister hooded rats .
### Output Text:
In this investigation the behavioural effects of chronic ( two week ) treatment with <span class="chemical">5 - FU</span> and ( three weeks ) with <span class="chemical">Fluoxetine</span> either separately or in combination with <span class="chemical">5 - FU</span> were tested on adult Lister hooded rats . 