Combine cereal, peanuts and pretzels in a roasting pan and toss to mix well. Combine brown sugar, margarine and corn syrup in saucepan. Bring to a boil, stirring occasionally. Boil for 1-1/2 minutes, stirring occasionally. Pour over cereal mixture, stirring until coated.
Bake at 350° in a preheated oven for 15 minutes, stirring 3 or 4 times. Spread cereal mixture in a thin layer on a sheet of wax paper. Let stand until cool.