The effect of a recent hyperammonemic model, consisting of a high ammonia diet for 3, 7, 15, 45, and 90 days, on glial fibrillary acidic protein (GFAP) in the rat spinal cord and on blood ammonia levels has been studied. The high ammonia diet was prepared by mixing a standard diet with ammonium acetate (20% wt/wt); in addition, 5 mM of ammonium acetate was added to the water supply. GFAP contents were determined by means of immunoblotting analysis. The results demonstrated that this high ammonia diet model neither induces significant changes in GFAP immunoreactivity, nor modifies total protein concentration, and only induces significant blood hyperammonemic levels in the first days of treatment. An adaptive response to the diet is suggested and discussed to explain these results. A relation between ammonia and GFAP expression is suggested because transient hyperammonemia induces transient, although no significant, changes on GFAP expression.