Dissolve soda in buttermilk, stirring well; set aside.
Sift together dry ingredients in a large bowl; make a well in center of mixture. Combine buttermilk mixture, peanut butter, and eggs; mix well. Add to dry ingredients, stirring just until moistened.
Spoon batter into a greased 8 1/2- x 4 1/2- x 3-inch loaf pan. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes. Remove from pan, and cool completely.