Javascript for populating text field based on dropdown list

Hi,

I am trying to write a javascript to populate a "Textfield" based on what I select on a "Dropdownlist" this has a one on one relationship with each other. I tried to embed the script on "Change" category and the script was as follows but it did not work. I'd appreciate if someone can help me with the correct procedure.

Thanks,

Vijay

form1.#subform[0].DropDownList2[0]::change - (JavaScript, both)

function populateDescription(){

        if (document.subform[0].DropDownList2[0].value=="BIH"){

                    document.subform[0].TextField83[0].value="Burn in and PBI History";

        }

}