The induction of behavioral sensitization of morphine in rat. Two groups of rats were exposed to morphine or saline for 4 consecutive days. Rats were given morphine or saline once daily on day 1 and twice daily on days 2–4 as described previously. The locomotor activity was monitored for 3 h in the chambers after the first injection once daily. Each point represents the total distance traveled for the 3 h test period. Data are shown as means ± S.E.M (n = 8 per group). * P < 0.01 compared to the first administration within group; # P < 0.05 and ## P < 0.01 compared to the corresponding saline control group.