First, have you had a physical checkup recently? (my caveat...beware of medical advice from strangers on the Internet) My guess is that you might have a mineral or vitamin deficiency...possibly iron. You might also pay attention to what you have for breakfast or the meal before your altar duty. Is it well-balanced? Are you careful not to have too many sweets and caffiene? Are you prone to anxiety attacks? Possibly the act of being in front of an audience is also at work?

suck on a hard candy, your blood sugar is probably low.

There is a common syndrome in soldiers standing at attention in which their blood pools in their legs and does return to their heart properly to get re-oxygenated. Eventually, your blood becomes so oxygen-poor that your brain isn't getting enough oxygen, and you faint. The "cure" is to not lock your knees when you are standing for long periods. I hope this is helpful; here are some stories about the problem. Two Tampa Bay Police Honor Guard Members Faint http://www.sptimes.com/2005/05/11/Tampabay/Two_officers_faint_du.shtml Absolute Writers Forum (go to top of page) http://www.absolutewrite.com/forums/showthread.php?p=498436 However, the other suggestions you have gotten should be considered. Some altar servers fast before Mass -- if you are doing this, it may cause low blood sugar which could make you faint. If so, talk to the priest about it; maybe your fast could be limited. Or, if you are extremely shy, you could be having a mild panic attack which causes a "vasovagal reaction," leading to dizziness and fainting. But I think the most likely thing is, you're standing a long time, possibly in hot garments, and you're getting soldiers' syndrome: boom, down you go! You can hurt yourself while falling, so this is important. Good luck; hope one of these explanations works out for you.

I get dizzy/nauseated if I kneel too long. I've been told by a nurse I know that lower-than-usual blood pressure probably has something to do with it.

sit down quick or grab hold of something if you can do it without hurting anyone else (are you carrying anything like a candle) head between knees. Sometimes I make sure I have a glass of water near where I sit, and it gives you a great excuse to raid the kitchen (if you have one) for a biscuit?

make sure you eat a good breakfast those candles burn up a lot of oxygen in the air and sometimes incense can make you queasy or naseauous.