Preheat oven to 350ºF. Stirring frequently, melt peanut butter in a small saucepan over medium heat. Remove from heat. In a large bowl, combine cake mix, pudding mix, water, eggs, and oil. Beat at low speed of an electric mixer for 30 seconds. Beat at medium speed for 2 minutes. Add 1 1/2 cups of batter to peanut butter. Pour remaining batter into a 10-inch tube pan. Spoon peanut butter mixture over batter. Bake 50 to 55 minutes, or until a toothpick inserted into center of cake comes out clean. Cool in pan 15 minutes. Remove from pan and cool completely on a wire rack. Serves 10-12.