Protected Sub ddlDemo_PreRender(ByVal sender As Object, ByVal e As _
  System.EventArgs) _
	 Handles ddlDemo.PreRender
		'An die Dropdonwliste ein JavaScript onchange event dranhängen
		'das die Javascript-Function aufruft, die
		'den aktuell selektierten Text in die Textbox einträgt
		Dim script As String = "SetTextBoxValue(this, '{0}');"
		script = String.Format(script, txtDemo.ClientID)
		ddlDemo.Attributes("onchange") = script
	End Sub

<script language="javascript">
function SetTextBoxValue(ddl, idTextBox)
{
	var txt = document.forms[0][idTextBox];
	txt.value = ddl.options[ddl.selectedIndex].text;
}
</script>