Rats were fed standard (20% protein), protein-free or high protein (80%) diets for 15 days and then injected intraperitoneally with ammonium acetate (7 mmol/Kg). Survival was 6%, 75% and 100%, respectively, for rats fed standard, protein-free and high protein diets. After injection of 6 mmol/Kg of ammonium acetate, blood ammonia reached a peak (at ca. 2 mM) after 7, 25 and 30 min for rats fed high protein, protein-free and standard diets, respectively. The results presented indicate that protection in the high protein group is due to faster detoxication of ammonia via a more active urea cycle while the tolerance of the protein-free group to higher levels of ammonia remains to be clarified.