Question Re: Treatment Of Mortgage Payments For Tax Purposes

Say, you have a $100K mortgage that you pay on each month...Do you take the interest deduction only? Take interest deduction, while writing off principal portion as a business expense? Write off the entire monthly payment as a business expense? We are in the process of looking for an experienced accountant, but I'd like to have a little background info. on this issue beforehand, if possible. Thx, JSJ