Acute hyperammonemia, induced by two consecutive injections of ammonium acetate (550 and 450 mg per kg b.wt.), decreased the activity of gamma-glutamyl transpeptidase (GGT) in most brain regions of 18- and 30-day-old rats. This decrease in the brain GGT activity was more pronounced in younger than in older rats. After the addition of NH4Cl to the incubation medium, the inhibitory action of NH4+ on this enzyme activity was also demonstrated in crude synaptosomal membranes at pH 7.4, but in a range of NH4+ concentrations many-times higher than those found in the plasma or brains of young hyperammonemic rats. Because similar concentrations of NH4+ stimulated the activity of the purified enzyme from rat kidney (mainly at pH 9.0), the inhibition of GGT activity in the young rat brain is probably mediated indirectly and not by a direct interaction of ammonia with the enzyme molecules.