Trying to nat with iptables (nat / prerouting / iptables)

I'm trying to doing something simple with iptables: redirect one port
of my firewall machine to other port at other machine.

This is what I do:

iptables -t nat -A PREROUTING -p tcp --dport 81 -j DNAT --to 192.168.1.3:139

This is not a real example (I am really trying to do this with vnc
port), but here I don't have vnc, so I am testing with this.

In the FORWARD chain my policy is ACCEPT, so I assume that the packet
must go to 192.168.1.3:139 (I am logging the packets in FORWARD, and
there is no traffic).

I assume that 192.168.1.2:81 must be equivalent to 192.168.1.3:139, but:

merry:~# telnet 192.168.1.2 81
Trying 192.168.1.2...
telnet: Unable to connect to remote host: Connection refused

merry:~# telnet 192.168.1.3 139
Trying 192.168.1.3...
Connected to 192.168.1.3.

What I am doing wrong? Thank you very much,
  Carlos.