How can you keep you weight constant while increasing muscle mass?

I have been steadily decling my weight from 172 to 160 lb in 2 months.  But I just started lifting weight to increase some muscle mass. As a results, I am started gaining weight again.  I am back to 163 lb again, and I assume that those extra pounds are due to increase in muscle mass.  How can I loose fat weight and increase muscle weight at the same time.  Is it okay to be 5-7 lb overweight from your normal BMI if you are muscular.  Please suggest.

If you've only just started lifting, those 3 pounds are probably actually water retention in your muscles as opposed to muscle tissue - though, since you're a guy (I'm assuming from your avatar), it might be a mixture of both.  You'll find out in a month or so (which is how long it takes for the muscles to let go of the water).

To maintain weight but gain muscle, you'd want to eat maintenance calories, make sure you're getting enough protein (about 1 gram per gram of lean mass, if I recall correctly) no more than 30 g at a time and lift heavy.  If you eat a couple of hundred calories more than you burn, you'll have an easier time gaining muscle but will likely get a little fat with it too.

Regarding the BMI question, it really depends on your body type and how much muscle we're talking about.  Certainly, some very muscular people are "overweight" but not really.  If you check your bodyfat percent, that'd give you a better idea of where you're at and if the extra weight is muscle.  If it is, you're fine.

Finally, don't be too shocked if your thread gets moved to the Fitness forum.  Weight Gain is mostly for people aiming to gain weight (as opposed to those observing a recent weight gain).

If your muscles are sore then there will be swelling.  Swelling retains water which makes you heavier on the scale.  Don't worry about it.

I think susiecue meant 1 gram of protien per Kilogram of body weight.  That is sufficient for moderate weight training.

I carry a lot of muscle, BMI for me is totally useless.  If I had 0% bodyfat, my BMI would still be high.

I can get good muscle tone without a ridiculous amount of work, and as a result, I look smaller than what I weigh. Even though according to the BMI scale, I need to lose about 2-3 more lbs. I plan on losing 10-12 more, but for people who work out a lot, body fat % and measurements are more telling. You might wanna get that tested (most gyms do it for free) and not worry about BMI so much. It will give you a better idea of whether or not you are gaining muscle mass too.