Effects of single administration of baclofen on the acute morphine stimulation of locomotor activity. Four groups of rats were exposed to morphine and one group of rats was given saline. For morphine treatment group, saline or baclofen (1.25, 2.5, 5 mg/kg, s.c.) was administered 15 min prior to the morphine injection (b). Locomotor activity was measured for 3 h on day 1 immediately after morphine administration. Each bar represents the total distance traveled for the 3 h test period. a: the saline/saline group and morphine/saline group. Data are shown as means ± S.E.M (n = 8 per group). ## P < 0.01 compared to saline/saline group; * P < 0.05 compared to morphine/saline group.