Limit on updates made in a transaction

Hi,

Is there is any limit on the number of updates made to a
postgreSQL database in a single transaction? If yes, how
can we get around this problem? For example, we need to
update a field of many existing rows in a table triggered
by the addition of a new row. If anything fails in the
update process, we'd like to undone everything (including
the addition of the new row).

Thanks in advance for any info!