Bit of a ramble here but I would really appreciate some advice. We have two Hungarian Vizslas.  Zolti is 4yrs old, Kazmer is 2.  We never had a hint of a problem with Zolti before getting the second one but ever since Kazmer arrived, Zolti has become increasingly aggressive towards other male, intact dogs when they're out walking.  (Some sort of protective instinct it seems like)   There have been several unpleasant but not serious incidents where he's got into a fight but it seems to be getting worse.  Yesterday he went for a golden retriever, completely unprovoked and both dogs had significant damage.  He's the gentlest soppy dog at home which makes it all difficult to understand.  We are now seriously considering getting him castrated (haven't done before because we showed him) My question is do any of you know if this actually helps in reducing aggressive tendencies or is there something else we should try?  Our vet says that "it sometimes helps but the decision is yours" which is probably true but not desperately helpful.

Our collie-cross rescue was dog aggressive. We had him castrated because it was part of the agreement with the rescue society. It made no difference whatsoever, neither did training classes. I assume he was inadequately dog socialised as a puppy. We sadly had to resort to never letting him off his lead. Sorry, I'm sure someone will have a more positive story.
Cathy.

Hi Guest,
  I think Zolti needs to learn that he is not the one in charge here, you are, and it's not up to him to protect Kazmer.  Do you do any sort of training with him?  I would start by lowering his status at home, and keep him on lead when other dogs are about, so that you can stop him having a go before he has the chance (and let him know you mean it!).  If I say to my dog 'leave it', when he wants to chase another dog, he will come straight back to me, and be rewarded with a toy or treat - so he has learnt (and not overnight I might add!), that it is more rewarding to come back to me than go off after another dog.  I doubt castration will help - my personal experience is of having a dog castrated at just over a year old, and he got more and more aggressive towards other male dogs - apparently once the testosterone hormones are no longer as strong as in an entire dog, it lowers their confidence, and it can become a fear thing - which you don't want either.
  Hilda

Whilst castration (in theory) reduces agression through reducing (in fact eliminating) testosterone, it is possible the agression is not induced by the presence of this hormone, but by a feeling of 'inadequacy' - the dog is feeling threatened already - and reducing the testosterone can actually amplify that feeling, leading to a MORE agressive dog.  Did you vet suggest 'chemical castration' (Tardak)?  This can help to identify wether it is testosterone which is driving your dog, or not......