Found this:
Chocolate contains theobromine and caffeine, which are both methylxanthines. The amount of methylxanthines present depends on the type of chocolate (see chart below.) In general, the less sweet the chocolate, the more toxic it could be. In fact, unsweetened baking chocolate contains almost seven times more theobromine as milk chocolate.
Methylxanthines can cause CNS stimulation, tachycardia, and tremors. Signs seen with chocolate toxicosis include vomiting, diarrhea, hyperactivity, polyuria, polydipsia, lethargy, tachycardia, cardiac arrhythmias, seizures, and death. Based on ASPCA Animal Poison Control Center (APCC) experience, mild signs occur in animals ingesting 20 mg/kg and severe signs are seen at doses over 40mg/kg.
Type of Chocolate Methylxanthines-mg/oz (approx):
Milk chocolate 50-62
Semi Sweet 160
Baking chocolate 428-440
Estimates of the smallest amounts that may be fatal are:
**Small dogs such as Chihuahua and toy poodles is 4-10 ounces of milk chocolate or 1/2 to 1 ounce of baking chocolate
** Medium dogs such as dachshunds and cocker spaniels is 1 to 1-1/2 pounds of milk chocolate and 2-3 ounces of baking chocolate.
** Large dogs such as Labradors and Collies is 2 to 4-1/2 pounds of milk chocolate or 4 to 8 ounces of baking chocolate.
When people call and want to know if the hershey's kiss their lab ate is going to kill it, I tell them about this to put it in perspective.
Margaret Moorman: In calculating a dose, I would recommend using the worst case scenario. For example, where it says milk chocolate 50-62 mg/oz, I would use 62 unless it has something else on the label.
Elizabeth Warren: Definitely Margaret, but a lot of people seem to have gotten the idea that chocolate=death, period.
Brenda: I had a 55# Bassett mix that ate a whole batch of no bake oatmeal chocolate cookies. Did not affect him at all. What is in a Hersey chocolate bar?
Elizabeth Warren: I get a lot of candy calls (milk chocolate). They really need to eat a bunch to become toxic. Brenda, I think they make baking chocolate in bars as well as candy bars (milk chocolate).
Margaret Moorman: Hershey's milk chocolate bars contain 55.5 mg/ox of theobromine and 6.45 mg/oz of caffeine. If a 5# dog ingested a 1.55 oz. chocolate bar, the dose would be 42.3 mg/kg.
Elizabeth Warren: Even small amounts of chocolate can and do cause GI upset. Yep; the smaller the dog, the smaller the dose to toxicity.
http://www.vspn.org/Library/Misc/VSPN_M02714.htm