Peanut Butter Marshmallow Treats

4 tablespoons butter or margarine
4 tablespoons peanut butter
10 ounces marshmallows
5 cups toasted rice cereal
non-stick cooking spray

1. Place butter or margarine and peanut butter in a large saucepan.  Cook on low heat until completely melted, stirring constantly.
2. Add marshmallows.  Continue to cook and stir until marshmallows are completely melted. 
3. Stir in cereal and mix until completely blended.
4. Spray a square baking dish with non-stick cooking spray.
5. Spread cereal mixture evenly into the pan.  Cool and cut into squares.