Fig. 3. Intrathecal morphine produces a dose-dependent naloxone-reversible antinociceptive effect at all postnatal ages at doses that have no effect after systemic delivery. Mechanical withdrawal thresholds 30 min after intrathecal injection are represented as a percent change from baseline threshold in postnatal day (P) 3 (A ), P10 (B ), and P21 (C ) rats. Mechanical threshold is significantly increased after doses of 10 and 30 μg/kg intrathecal morphine in P3 and P10 rats, respectively, and 150 μg/kg in P21 rats. Effects of intrathecal morphine are antagonized by coadministration of naloxone (nal), and the maximal dose given subcutaneously (sc) has no effect on mechanical threshold. *P < 0.05, **P < 0.01 morphine vs.  saline; # P < 0.01 naloxone plus maximal dose intrathecal morphine vs.  maximal dose intrathecal morphine; §P < 0.01 subcutaneous vs.  intrathecal administration of maximal dose morphine. Bars = mean ± SEM; n = 5–12 per group; one-way analysis of variance with Bonferroni multiple post hoc  comparison with saline.