Fig. 1. Effects of codeine and morphine on mechanical sensory thresholds for Sprague-Dawley rats (the extensive metabolizer group). Time course of the effect on postnatal days 21 (P21, a ); 10 (P10, b ); and 3 (P3, c ). (d ) Peak effects on P3 for morphine and codeine. **= highly significant (P < 0.001). vFh = von Frey hair.