OBJECTIVE To investigate the effects of surfentanyl and fentanyl on spontaneous contractions of isolated uterine smooth muscles of rabbits. METHODS Thirty isolated uterine smooth muscles strips of rabbits with rhythmic spontaneous contraction were randomly divided into three groups: control group (n = 10) fentanyl group (n = 10) and surfentanyl group (n = 10). The effects of fentanyl and surfentanyl on spontaneous contractions intensity, frequency and duration of isolated uterine smooth muscles of rabbits in different concentrationswere monitored. RESULTS There were no effect on the spontaneous contraction of isolated uterine muscles of rabbits at the concentration of 1 x 10(-8) mol/L in fentanyl group. The spontaneous contraction intensity was decreased significantly when the concentration of fentanyl more than 3 x 10(-8) mol/L (P < 0.05). The spontaneous contraction intensity, frequency and duration was inhibited more than 50% when the concentration of fentanyl more than 3 x 10(-7) mol/L (P < 0.05). And the spontaneous contraction duration was inhibited more than 50% when the concentration of surfentanyl more than 1 x 10(-7) mol/L (P < 0.05). Compared with surfentanyl group, there were significant difference on spontaneous contraction intensity when the concentration of fentanyl more than 3 x 10(-8) mol/L (P < 0.05). Difference on frequency, more than 3 x 10(-8) mol/L (P < 0.05) and difference on duration, at the concentration of 1 x 10(-7) and 1 x 10(-7) mol/L (P < 0.05). CONCLUSIONS Fentanyl restrained the spontaneous contractions of isolated uterine smooth muscles of rabbits in concentration-dependent manner. Surfentanyl could reduce the spontaneous contractions duration on isolated uterine smooth muscles of rabbits. But there were no effect on frequency and duration.