I am not a vet but found this on the internets. I thought it was the caffeine in chocolate so this was informative for me. If your pup hasn't passed out you are probably ok but always best to phone the emergency vet/poison control (pets) or your animal critical care center, depending upon your resources.
http://www.answerbag.com/q_view/129497"Estimates of the smallest amounts that can be fatal are:
4 ounces of milk chocolate or 1/2 ounce of baking chocolate for small dogs, such as Chihuahuas and toy poodles.
8 ounces of milk chocolate or 2 ounces of baking chocolate for medium-sized dogs, like cocker spaniels and dachshunds.
8 ounces of milk chocolate or 4 ounces of baking chocolate for large dogs, including collies and Labrador retrievers.
Cats have much different eating habits and seldom are poisoned by chocolate.
While a very small amount of chocolate may not harm some dogs, it's safest to avoid giving it to them at all. If an accident occurs, a veterinarian should be consulted. Treatment may require inducing vomiting, stabilizing the animal's heartbeat and respiration, controlling seizures and slowing the absorption of Theo bromine. If the animal already is comatose, its stomach may need to be pumped.
Theo bromine does not completely leave the dogs system, as do some foods, my research informs me that Theo Bromine has a half life of 17.5 hours. This means that half of it has gone in 17.5 hours; in 35 hours another half of the half that was remaining will have gone. We can go on and on forever and ever getting less and less, to be silly about it, it never gets to zero!"