Postoperative sensitivity to tactile stimuli differs as a function of age. In this study, we hypothesized that preoperative sciatic nerve block (SNB), by providing preemptive analgesia, would result in better analgesia than postoperative SNB in the young rat. With the paw incision model of postoperative pain, male Sprague-Dawley rats, aged 2 or 4 wk, underwent general anesthesia and then received a left SNB with 5 microL/g of 0.5% bupivacaine or normal saline. SNB was performed either before or after surgery. Mechanical allodynia was assessed by using von Frey filaments before and at various times after SNB and surgery. In the 2-wk-old rats, preoperative SNB produced a significant reduction in mechanical allodynia, as reflected by a higher threshold at 2, 5, and 24 h when compared with saline control (P < 0.03). At 24 h, the threshold was 4.0 +/- 0.7 g in the preoperative SNB group compared with 1.6 +/- 0.3 g in the postoperative SNB group (P = 0.004). There was no difference at any time point between the preoperative and the postoperative SNB in the 4-wk-old animals. These results suggest that preoperative SNB in young animals provides a preemptive analgesic effect on mechanical allodynia that is age or developmentally dependent.