Preheat the oven to 350 degrees F (175 degrees C). Grease two 9x5 inch loaf pans.
In a large bowl, stir together the whole wheat flour, all-purpose flour, brown sugar, baking soda, and salt. Pour in buttermilk, and stir until all of the dry mixture has been absorbed. Divide the batter evenly between the prepared loaf pans.
Bake for 1 hour in the preheated oven, or until a knife inserted into the crown of the loaf comes out clean. Serve warm. Store leftover bread wrapped in plastic wrap in the refrigerator. This reheats well.