How to cast VARCHAR to BYTEA and vice-versa?

Hello all,
I have a table with a VARCHAR column that I need to convert to a BYTEA.
How do I cast VARCHAR to BYTEA?
The following doesn't seem to work as it yields the 'cannot cast varchar to
bytea' error message:   varchar_data::bytea
On the same topic, how do I do the reverse, that is, how to I cast from
BYTEA back to VARCHAR?
Thanks,
-Michael Moran