Fig. 4. Latency of withdrawal in seconds from noxious heat for Sprague-Dawley rats (the extensive metabolizer group) on postnatal days 3 (P3), 10 (P10), and 21 (P21). Open bars = morphine 2 mg/kg; filled bars = codeine 10 mg/kg; gray bars = saline; **= highly significant (P < 0.001).