Script export problem in FC3

[Samuel Díaz García]

I need to modify the PATH environment using a script. My script is:

--begin--
#!bin/sh
export PATH=$PATH:/mydir
--end--

If I run the export command in command line I have no problem, but when 
I run my script, and I run

echo $PATH

I can see that the path environtment variable isn't changed.

What can be the problem?
I'm root user when I run the script.

Thanks