changing select options from list menu

Please help!!
how do I change the selection options from a list menu based on the option
selected from a second list menu?

list menu 1= depcity
list menu 2= retcity

If Las Vegas is selected from depcity, then retcity menu would list all
If Las Vegas is not selected from depcity, then retcity menu would show Las
Vegas as the only selection




<select name='dcity' size='1' id="depcity" value=''>
<option value="" selected>- Departure City
-</option>
<option value="BLI">Bellingham, WA</option>
<option value="BIS">Bismarck ND</option>
<option value="CID">Cedar Rapids, IA</option>
<option value="COS">Colorado Springs, CO</option>
<option value="DSM">Des Moines, IA</option>
<option value="FNL">Fort Collins, CO</option>
<option value="FAT">Fresno, CA</option>
<option value="LAN">Lansing, MI</option>
<option value="LAS">Las Vegas, NV</option>
<option value="MSN">Madison, WI</option>
<option value="PIA">Peoria, IL</option>
<option value="RDD">Redding, CA</option>
<option value="FSD">Sioux, SD</option>
<option value="ICT">Wichita, KS</option>
</select>

<select name='rcity' size='1' id="retcity"
value=''>
<option value="" selected>- Arrival City
-</option>
<option value="BLI">Bellingham, WA</option>
<option value="BIS">Bismarck ND</option>
<option value="CID">Cedar Rapids, IA</option>
<option value="COS">Colorado Springs, CO</option>
<option value="DSM">Des Moines, IA</option>
<option value="FNL">Fort Collins, CO</option>
<option value="FAT">Fresno, CA</option>
<option value="LAN">Lansing, MI</option>
<option value="LAS">Las Vegas, NV</option>
<option value="MSN">Madison, WI</option>
<option value="PIA">Peoria, IL</option>
<option value="RDD">Redding, CA</option>
<option value="FSD">Sioux, SD</option>
<option value="ICT">Wichita, KS</option>
</select>
</font></div></td>
</tr>