custom web control +client-side javascript + postback

Hello,

I am trying to create a new Web Control (ASP.NET 1.1) that contains
among other textboxes. The content of these textboxes (runat=server)
might be changed by client-side Javascript code, like:
document.getElementById('txt...').value='...';
Then, at server side, I will need to retrieve the NEW content of these
textboxes later on...
The problem I am facing is that I don't retrieve the new content of the
textboxes... It seems that after postback, I don't get these new
values...

Could someone explain me how to achieve this?

Many thanks