Hyperammonemia produced by an infusion of glycine in dogs leads to death within four hours. Concurrent treatment with a total dose of 15 or 20 mg/kg of diphenhydramine hydrochloride (Benadryl) significantly reduces the mortality to 12%. Although the surviving animals have lower average peak ammonia levels, there is no dose-specific relationship between the dose of diphenhydramine and the level of hyperammonemia. The mechanism of action of diphenhydramine remains unanswered.