I already made it, but nevertheless THANKX.

I created the following if statement within the html_record_form which does the things I wanna to have. I did this for every field which should be handelt in this way.

if ( $rec{'Participant 1'} eq "") # Is the field empty ??
{
print qq| <TR><TD ALIGN="Right" VALIGN="TOP">1.Participant/Dept./Phone:</TD>
<TD VALIGN="TOP">&nbsp;<INPUT TYPE="TEXT" NAME="Participant 1" VALUE="$rec{'Participant 1'}" SIZE="40" MAXLENGTH="40"></TD></TR>
|;
} else {
print qq| <TR><TD ALIGN="Right" VALIGN="TOP">1.Participant/Dept./Phone:</TD>
<TD>&nbsp;$rec{'Participant 1'}</TD></TR><INPUT TYPE=hidden NAME="Participant 1" VALUE="$rec{'Participant 1'}">
|;
}