The quaternary amine L-carnitine is able to protect Swiss Albino mice from hyperammonemia when administered in high doses before ammonium acetate. This has been explained by its specific ability to shuttle fatty acids into mitochondria. The structure of L-carnitine resembles the chemical structure of other substances that have been described as being able to protect living cells against osmotic stress. We subjected Swiss Albino mice to hyperammonemia after pretreatment with L-carnitine or "osmoprotectants" such as the quaternary amines choline and betaine, and trimethylamine N-oxide. L-Carnitine proved to be the drug of choice to protect against acute hyperammonemia. Nevertheless, the other tested compounds appeared also to be effective, suggesting that osmoregulation plays a major role in protection against hyperammonemia.