You don't want to pass the parameter as per below example. Current element value will be taken as parameter to controller through POST.
http://www.yiiframework.com/wiki/24/creating-a-dependent-dropdown/
echo CHtml::dropDownList('country_id','', array(1=>'USA',2=>'France',3=>'Japan'),
array(
'ajax' => array(
'type'=>'POST', //request type
'url'=>CController::createUrl('currentController/dynamiccities'), //url to call.
//Style: CController::createUrl('currentController/methodToCall')
'update'=>'#city_id', //selector to update
//'data'=>'js:javascript statement'
//leave out the data key to pass all form values through
)));
//empty since it will be filled by the other dropdown
echo CHtml::dropDownList('city_id','', array());
If you want to send data manually, then you have to uncomment the 'data'=>'js:javascript statement'
.