You must LINK to the tables in the other database. From the Menu,
choose File > Get External Data > Link. Once that is done, you can
treat the external table as if it were in your original database.
Actually, this is not a helpful response to the original question. It is
_not_ possible to create relationships between linked tables.
If you think about it for a moment, the reason is clear. If a a table
inside A.mdb is referenced by foreign key in a table inside B.mdb, then a
user using A.mdb could delete a row that is still referenced, because B.mdb
is not open. Even worse, a user could delete the whole file A.mdb, thus
immediately rendering the other table in B.mdb invalid.
The answer is, I am afraid, that relationships can only be declared and
enforced within the same database.
B Wishes
Tim F