Add private key permanently with ssh-add on Ubuntu

Question

I have a private key protected with a passowrd to access a server via SSH.

I have 2 linux (ubuntu 10.04) machines and the behavior of ssh-add command is different in both of them.

In one machine, once I use "ssh-add .ssh/identity" and entered my password, the key was added permanently, i.e., every time I shutdown the computer and login again, the key is already added.

In the other one, I have to add the key every time I login.

As far as I remember, I did the same thing on both. The only difference is that the key was created on the one that is added permanently.

Does anyone know how to add it permanently to the other machine as well?

Thanks in advance