Rey's brain is a miscellaneous item in Fallout: New Vegas.
Characteristics
It is one of three possible replacement brains for Rex in the quest Nothin' But a Hound Dog. If it is used as a brain for Rex, he will have an increase in attack power.
Location
It can be bought from Old Lady Gibson at Gibson scrap yard for 700 caps (free with a Barter check of 70, or with the Lady Killer perk).
This item can also be gained by simply killing Old Lady Gibson's dog Rey yourself and taking it off of its body.