Objective    To study the influence of low intensity ultrasound on the contraction of uterine smooth muscle in rats in order to find suitable exposure parameters of the contraction.     Methods    The uteri were taken out and irradiated by ultrasound at different intensities and for exposure times (0.8 MHz

1-3 W/cm

2

) by the ways of orthogonal design. The contraction frequency and amplitude were recorded with BL-410 biological experimental system.     Results    The contraction frequency

amplitude

tension and activity were significantly increased during ultrasonic irradiation (P

<

0.01) compared with no irradiation. The increased contraction frequency and amplitude lasted for 10 min

and then the nor

mal contraction pattern was resumed. When the intensity of 2 W/cm

2

ultrasonic irradiation lasted for 10 min

the contraction frequency

amplitude

tension and contraction activity peaked during 2 W/cm

2

ultrasonic irradiation for 10 min.      Conclusion    Low intensity ultrasound can induce uterine smooth muscle contraction in rats. With constant frequency

the suitable parameter is 2 W/cm

2

ultrasonic irradiation for 10 min.