How can I delete a primary or foreign key?

I am using PostgreSQL 7.4.1 (only through psql)
I know, that the command

ALTER TABLE OFFICES
    DROP PRIMARY KEY (CITY);

and its foreign key equivalent:

ALTER TABLE SALESREPS
    DROP CONSTRAINT
FOREIGN KEY (REP_OFFICE)
    REFERENCES OFFICES;

don't work in PostgreSQL because they are not implemented. However, isn't
there another way of removing them?
I also tried to drop the index associated with the primary key, but it is not
permitted.

Anyone with any idea?
--
Tibor