This is not something we reccomend in our practice, but I've had several acquaintances try it on their own. As far as I know it hasn't worked for any of them. If the testicle is truly retained in the abdomen then the HCG is not likely to work.
Were both testicles present as a weanling and now one has gone back up? If so, chances are it will eventually drop back down again. I've heard mixed reviews about using HCG, and have heard it working on several colts. But like Hillside said, if the testicle is up in the abdomen, no HCG in the world is going to bring it back down, only surgery!
Don't know if the testicle was ever present as the colt was not with me until recently. I realize if the testicle is really up in the abdominal cavity nothing will help but, I'm hoping it just needs a little kick to bring it down from the inguinal ring area. I read that the majority of testicles that respond to treatment and are lower placed are on the right side and that's the one on my colt that's "missing". I'm ready to have the surgery done but, figured this route was something worth trying before I spend the bucks on surgery.
I used it successfully in one of mine. I think it was once a week for six weeks maybe? Can't remember but it did drop. Once it was palpable we applied gentle downward traction (aka pulling on it) which of course he loved. I think he was coming 2yo at the time. Good luck!
Destiny is not a matter of chance, it is a matter of choice; it is not a thing to be waited for, it is a thing to be achieved. - William Jennings Bryan