In order to get a masters degree in a field, is it necessary to have a bachelor's in the same field? For example, if I get a business degree (BBA) from one school, and I get good marks and do well on the MCAT, could I apply and be admitted into Med School?

As far as I know undergraduate degrees are not necessarily a prerequisite for post graduate degrees. That being said, there may be some undergrad course requirements that need to be completed. For example you might need certain credits in Biology and Chemistry before entering med school.

The best place to ask would be your college's admissions department.

After reading Nez's reply I see I need to clarify mine. You do need a UnderGrad degree before pursuing a post grad degree. But the undergrad degree does not have to be in the same field as the post grad. So you could get a BA in Poli Sci, then go onto to med school. Its not the usual way and there may be some course prerequisites you need to get, but its possible.

Hi, Student,
I have two Bachelors Degrees and a Masters Degree, but not in medicine.
Here is a site listing the most common 6 courses required for entrance into Medical Schools:
http://www.studentdoc.com/medical-school-requirements.html

No, you don't need a Bachelors Degree in the same field as a Masters Degree; but it does depend on the Masters Degree prerequisite courses, and as a rule-of-thumb, are normally found in the course content for a Bachelors Degree, leading up to a Masters in the same field.
The MCAT results are VERY important. Check with the Med School you are considering, or call them, and they will mail you a listing of their required prerequisite courses for entrance. Also, check with the college counselors. They have information on different schools.

If you would like to email a school, asking about the prerequisites for admission, here is a link to the prestigious Mount Sinai School of Medicine (New York):
http://www.mssm.edu/bulletin/admissions/admissions.htm

Most medical schools do not even require that you have an undergraduate degree to be accepted. They only require that you have the prerequisite courses to do so. Most every graduate program would require that you have a Bachelors degree and in some instances even a Masters degree.