Rats were treated with METH (10 mg/kg i.p. every 2 hrs × 4) or saline (1 mL/kg i.p. every 2 hrs × 4) and Lactulose (5.3 g/kg p.o. every 12 hrs) or vehicle (8 mL/kg, every 12 hrs). At 2 hrs after the last injection of METH or saline, plasma ammonia levels are significantly elevated by 64.04±9.98% (Mean±SEM) and this increase is blocked by lactulose treatment (*= p<0.001 compared to Lac Sal, &=p<0.001 compared to Veh METH). (n=8 rats per group) Veh=Vehicle, Lac=Lactulose, Sal=Saline