Dear Matthew: We have a 3-year-old Dalmatian bought at a puppy age. He has been loving and goofy. He loves to play with dogs, chase the ball and play with kids and his new sister, who we just got 3 months ago, a new dalmatian puppy.He is well-trained, thanks to your books. He comes, sits, stays and listens to me on and off the leash. He goes to a dog park, which he has been playing in since a puppy, two-plus years ago. He is off the leash and has a ball with the other dogs.

In April of this year, he had urinary stones and had to have a few surgeries and a catheter, and he went through a painful 30-day ordeal. The doctors praised him and remarked about what a well-behaved and sweet dog he was, giving no fuss through it all. After he healed, he was back to his goofy self -- except for one thing. When a dog would come up and touch his back haunches, he would bare his teeth and growl. When this happened, I immediately said "NO" and he would back up and walk away.

This last 30 days, it has escalated to at least two altercations a week, where he has actually lunged at the dogs and tried to bite. Again, I say no, put him in a "time out" box to let him cool off, then bring him back into the park and he leaves the dogs alone.

Let me add he is not in any "physical" pain, so the other dogs are not hurting him in any way. His little sister, a puppy, climbs all over him and plays hard, and he doesn't go after her or bite her at all.

Yesterday, without being provoked, he went after a black lab and attacked him, biting him and getting bit. Now it's serious, and I am beside myself over this.

I cannot take him to the park now. I don't trust him to be off the leash. I don't know WHY this has happened or where the behavior came from. What to do now? What do I do? Please help us, we are just sick over this. He is a good dog doing a bad thing and we need help. -- Jules in Sausalito, Calif.

Dear Jules: It sounds like your dog is still suffering from some emotional trauma as a result of his surgery. (I'm assuming your dog has been neutered, so unchecked male hormonal aggression isn't a possibility.) You may never be able to find out exactly why he's acting the way he is. In some way, it seems, he is connecting the pain he went through to other dogs, and as a result he is displaying aggressive tendencies towards strange animals as a knee-jerk reaction.

Unfortunately, there is no quick-fix solution to this problem. It may take months and years of conditioning to get your dog to feel comfortable around unknown animals. On the bright side, at least your dog hasn't displayed any aggression toward humans -- and at this point, he probably isn't going to develop any.

What you need to do is limit your dog's interaction with strange animals to controllable situations. Don't take him to a dog park when you know there are going to be lots of others there -- wait for the more quite time, when just a few animals are present. Make sure your dog is wearing a training collar, and continue giving him the strong corrective jerk whenever he starts losing control. Avoid any close encounters with other dogs in the beginning, and take your dog home if he looks like he's going to begin getting aggressive again.

Keep this up and see how your dog's behavior changes. With time, the freshness of the surgery in your dogs mind will diminish, and he may start letting other dogs near him. If that doesn't happen, I'd recommend you consult with a professional dog trainer for some one-on-one advice.

And here's a note to the rest of you: If your dog is due for surgery, major or minor, be sure he or she comes home to a quiet environment to recuperate. Your dog is going to be tired and possibly grouchy, so the more you can do relax and comfort the animal, the better.

Matthew Margolis is the host of "Woof! It's a Dog's Life," a dog-instruction series airing every Saturday on your local PBS station. Read all of Matthew Margolis' columns at the Creators Syndicate Web site www.creators.com, and visit him at www.matthewmargolis.com. Write him at 5777 W. Century Blvd., Suite 700, Los Angeles, CA 90045.