Figure 2. Ketamine given in a subcutaneous dose of 10 mg/kg (KET - 10) prevents the rightward shift in the morphine (MOR) dose-response curve on day 4. Tolerance, as assessed by a rightward shift in the cumulative dose-response curve, was produced by administering MOR three times a day subcutaneously at 10 mg/kg on day 1, at 20 mg/kg on day 2, and at 40 mg/kg on day 3. KET - 10 or saline (SAL) was administered 15 min before and after each MOR dose. On day 1, the first dose of KET - 10 or SAL was followed by a cumulative MOR dose-response assessment. On day 4, each animal underwent cumulative dose-response assessment without previous KET - 10 or SAL administration. On day 4, the SAL + MOR curve shifted more than five times, but the curves for the day 1 KET - 10 and day 4 KET - 10 + MOR groups were not significantly different from the day 1 SAL group (see Table 1for ED50values).