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). During drug treatment, temperature was measured one hour after each METH injection (denoted by arrows). METH treatment caused significant hyperthermia in both vehicle+METH and lactulose+METH treated rats, and lactulose had no significant effect on hyperthermia. (*=p<0.001 compared with Veh Sal) (n=6–9 per group) Veh=Vehicle, Lac=Lactulose, Sal=Saline