Effects of chronic treatment of baclofen on the development of morphine sensitization. Four groups of rats were exposed to morphine and one group of rats was given saline for 4 consecutive days. For morphine group, saline or baclofen (1.25, 2.5, 5 mg/kg, s.c.) was concomitantly administered 15 min prior to the morphine injection. During a 3-day withdrawal period, rats in morphine group were still injected with saline or baclofen (1.25, 2.5, 5 mg/kg, s.c.) daily (b). On day 8, all five groups of animals were subjected to an acute challenge of morphine (3 mg/kg, s.c.). The locomotor activity was monitored for 3 h in the chambers. Each bar represents the total distance traveled for the 3 h test period. a: the saline/morphine group and morphine/saline group. Data are shown as means ± S.E.M (n = 8 per group). ## P < 0.01 compared to saline/morphine group; * P < 0.05 and ** P < 0.01 compared to morphine/saline group.