If you take the MCAT during your senior year, then you have decided to take a year off before going to med school. You need to take the MCAT during your junior year or early in the summer after your junior year in order to be competitive for applying for direct entry after college.
I took the MCAT in March of my Junior year. I was done with all of my pre-reqs (bio, chem, orgo, physics), and was taking second semester physical chem, advanced organic chem, and instrumental methods of analysis (another chem course).
