Figure 1. Experimental design. Drugs were administered intraperitoneally three times a week for 3 weeks. Amphetamine (AMPH) doses were increased weekly (1 to 3 mg/kg). N-acetylcysteine (NAC, 60 mg/kg) or saline (SAL) was given immediately before saline or amphetamine during the second and third weeks. Animals were returned to their home cages after injections. Behavioral tests started after a 4-week washout. Animals were assigned to the following experimental groups: SAL/SAL, NAC/SAL, SAL/AMPH, or NAC/AMPH.