Just wondering what should be done first. I joined a gym with a pool and swim straight 13 laps, then do various things to work out the legs and arms for the 2'nd half (total of 20 minutes), then I 'll do weights. Sometimes they have an auqa class in session from 9-10am and I have to use the pool AFTER the weights - which way is the better way? And is there a problem doing the weights first some days and the pool first other days? Weight is not my concern here, my concern is strengthening, good bone mass, and keeping a healthy heart. I keep getting conflicting reports on which is better to do first! (And no, I can't go later or earlier to miss that aqua class.)

Ideally weights first. Your body cannot use fat for fuel for anaerobic (weight) exercise, so if you burn off all your glucose doing swimming first, you risk actually losing some muscle doing weight training afterward. But your body is able to use fat during aerobic activity, so if you burn up glycogen/glucose with weight training, then your body can still tap into fat reserves for cardio later.

Wow, that makes sense! I never had it explained to me the WHY, thanks! That does even make it easier for me because I get to the gym at 9:15, by the time I do weights, get dressed, etc., the auqa class would be over and I could do the swimming! :)

Yep, weights first is correct. I used to be a fairly good competitive swimmer and we always did weight training before hitting the pool - weight training with no energy is not a good idea, as Naxis has stated.