RJS and Rails

Well i guess you use the wrong helper here. Remote-function doesn't
automacigally take the value of the selected option and sends it in
params[] i think. You took the code from the Rails API docs i assume.
Your controller code might be helpful too

You should try observe_field:

<select id="client_gender" name="client[gender]">
        <option value="">- Select -</option>
        <option value="Male">Male</option>
        <option value="Female">Female</option>
</select>
<% = observe_field "cliend_gender", {:url => {:action => change}, :on
=> "changed"}

This should take the "value" of the <select> tag and pass it in the
params hash