Did this happen in only one class, and how many credits was the lab portion of this class worth? If it is an isolated incident contradictory to the rest of your transcript, you'll be fine. However, you should have a stock explaination on hand for your interviewers at the med school b/c they'll likely ask you about it. Above all else-Don't let the F ruin your life or take away your motivation for other courses. You still have a good shot, and it's good that you retook the class. Just be careful not to make a habit out of it. I can sympathize w/ you to some extent because I always spend 2 or 3 times amount as much energy on my chem labs as I do on the lecture material even though they are worth less credit. Just keep your head up about it--very few applicants are perfect. Good Luck.