How to set "auto commit" off in postgresql db?

Hi:
 
It seems to me that postgresql db's default set for commit is "on". I would like to set it to "off". How could I do
that?
 
TIA.
----------
Here is what I got when testing:
 

webstore=> create table t1 (col1 varchar(10));
CREATE
webstore=> insert into t1 (col1) values ('test123');
INSERT 1006769 1
webstore=> select * from t1;
col1
---------
test123
(1 row)
webstore=> rollback;
NOTICE: ROLLBACK: no transaction in progress
ROLLBACK
webstore=> select * from t1;
col1
---------
test123
(1 row)
 
 Get your FREE download of MSN Explorer at http://explorer.msn.com