Problem with now() in function pgsql

Hi,
I have a problem in a function pgsql with Reading TIMESTAMP calling the function now()
the problem is this:
At the begin of function I read timestamp...
select now() into TS_BEGIN;

the function execute some statement...

Many seconds after I reread the timestamp
select now() into TS_END;

and TS_BEGIN is identical TS_END!!!

Can anyone help me?

Thanks in advance.

Luke.