Background The current study was designed to test the hypothesis that dexmedetomidine added to ropivacaine would increase the duration of antinociception to a thermal stimulus in a dose-dependent fashion in a rat model of sciatic nerve blockade. Methods Fifty adult Sprague-Dawley rats (10 rats/group) received unilateral sciatic nerve blocks with 0.2 ml ropivacaine (0.5%) or 0.2 ml ropivacaine (0.5%) plus dexmedetomidine (2.7 microm [0.5 microg/kg], 11.7 microm [2 microg/kg], 34.1 microm [6 microg/kg], or 120.6 microm [20 microg/kg]) in a randomized, blinded fashion. Time to paw withdrawal latency to a thermal stimulus for both paws and an assessment of motor function were measured every 30 min after the nerve block until a return to baseline. Results Dexmedetomidine added to ropivacaine increased the duration of dense sensory blockade and time for return to normal sensory function in a dose-dependent fashion (P < 0.005). There was a significant time (P < 0.005), dose (P < 0.005), and time-by-dose effect (P < 0.005) on paw withdrawal latencies of the operative paws. There were no significant differences in paw withdrawal latencies of the control paws, indicating little systemic effect of the dexmedetomidine. The duration of motor blockade was also increased with dexmedetomidine. High-dose dexmedetomidine (120.6 mum) was not neurotoxic. Conclusion This is the first study showing that dexmedetomidine added to ropivacaine increases the duration of sensory blockade in a dose-dependent fashion in rats. The findings are an essential first step encouraging future efficacy studies in humans.