See
WD2000: Text Form Fields Are Not Retained During Mail Merge
http://support.microsoft.com?kbid=211308

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Hope this helps,
Doug Robbins - Word MVP
> When using a Word document that contains both data merge fields and form
> fields (where you click, and then begin typing appropriate information),
> you
> cannot run a application.mailmerge.execute command. If you do, the form
> fields no longer function. I have identified this issue in Word 2000 and
> Word 2003.