Retrieve the primary key of a table

Hi. I need to retrieve the primary key of a table and their value.

I tried with this http://wiki.postgresql.org/wiki/Retrieve_primary_key_columns:

SELECT               
  pg_attribute.attname, 
  format_type(pg_attribute.atttypid, pg_attribute.atttypmod) 

FROM pg_index, pg_class, pg_attribute 
WHERE 
  pg_class.oid = 'TABLENAME'::regclass AND
 indrelid = pg_class.oid AND
  pg_attribute.attrelid = pg_class.oid AND 
 pg_attribute.attnum = any(pg_index.indkey);

But this shows indexes too. http://wiki.postgresql.org/wiki/Talk:Retrieve_primary_key_columns

How can I get only the primary key?

I have postgres 8.1.11.

Thanks, Alejandro.