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: Pretreatment with naloxazone significantly blocked morphine analgesia , catalepsy and hypothermia at a dose which completely eliminated high - affinity binding in brain membranes .
### Output Text:
Pretreatment with <span class="chemical">naloxazone</span> significantly blocked <span class="chemical">morphine</span> analgesia , catalepsy and hypothermia at a dose which completely eliminated high - affinity binding in brain membranes . 