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: Behavioral and cardiorespiratory responses to a lethal dose of morphine were evaluated in rats pretreated with saline or naloxazone , an antagonist of high - affinity mu 1 opioid receptors .
### Output Text:
Behavioral and cardiorespiratory responses to a lethal dose of <span class="chemical">morphine</span> were evaluated in rats pretreated with saline or <span class="chemical">naloxazone</span> , an antagonist of high - affinity mu 1 opioid receptors . 