If you are using Live 4 or 5 and Reason 2 or 3, you can send MIDI from Live's tracks to the devices in Reason via ReWire by following these steps:

1. Start Live, and then start Reason. This sets Reason up as a ReWire slave. Set up the modules in Reason the way that you want them.
2. If necessary, create a new MIDI track in Live. (From the Insert menu, select "Insert MIDI Track.")
3. Make sure that the View menu's "In/Out" option is checked.
4. To route MIDI from Live into Reason, choose "Reason" from the new track's "MIDI To" Output Type chooser.
5. Below the Output Type chooser, select the specific Reason device to which you'd like to send MIDI from the Output Channel chooser.