we own two male dogs who get along brilliantly and always have done,the thing is a few people have asked if they could use one of them as a stud dog.,but i am worried that it would change his temperament,he is so gentle with the kids and my other dog infact with everyone.ive heard that once used for stud a male is never the same is this true ?

In one word Yes

Sometimes things stay the same, sometimes they don't.  A friend had two males living happily, then one was used at stud and after that he wouldn't tolerate the other one - a real dominance issue.  Anyway, unless your dog is likely to get regular stud work, it's not worth the hassle for a one-off.  What they've never had they won't miss, but once they've had it, well ......

Also, please remember you are also responsible (morally) for resulting puppies, so don't even consider using your dog unless he has had all the health checks relevant to your breed?  Also, are you positive he is a good specimen of the breed and likely to produce good, typical puppies?

Think it depends on the individuals and the breed. I can honestly say it's never noticeably changed my dogs, but that's not true for all dogs. If you are doubtful, why take the risk?

It's not always the male used at stud which then causes the problems.

Someone I know has two male dogs - one, the lower ranking male, used as a stud more than once, and the other, very very dominant (owner's fault), not used at stud.  The dominant male must be able to tell/smell that the other has been studding, because his dominance over the other has got worse with every studding to the point that the other is subdued and depressed.