How can I display the contents of a function?

["Thom Brown" <thombrown@xxxxxxxxx>]

I would like to see what's in a function.  I can do this if I use pgAdmin III just by clicking on the function, and it appears in the SQL pane opposite.  But how is this done?  Is there a command like DESCRIBE FUNCTION logging.do_stuff; ?  And if possible, doing this without psql.

Thanks

Thom