The toxic principles of chocolate are the stimulants theobromine and caffeine. The amount of chocolate that it takes to poison your pet depends on the type of chocolate he's eaten and his weight. White chocolate has the least amount of stimulants and baking chocolate or cocoa beans have the highest. Once toxic levels are reached, signs of toxicity include: restlessness, hyperactivity, muscle twitching, increased urination and/or excessive panting. If your pet isn't treated, he could go into a seizure - possibly even die.
For more information, please read the article Chocolate Toxicity