Objective: To evaluate the effects of contrast-enhanced ultrasound (CEU) on the permeability of placental barrier primarily. Methods: A total of 60 pregnant Sprague Dawley (SD) rats were divided into 10 groups, including six groups of microbubbles-enhanced ultrasound (varied mechanical index (MI) of 0.13, 1.0 and 1.4 with continuous and intermittent insonation respectively) (US + MB), two groups of ultrasound insonation only (continuous and intermittent insonation respectively) (US), the group of microbubbles only (MB) and the control group. Evans blue (EB), as the tracer, was intravenously injected before treatment. The EB in placenta and fetus was observed under fluorescence microscope and analyzed quantitatively. The EB amount was compared between groups and between placenta and fetus. Lanthanum nitrate-tracing transmission electron microscope examination was performed to observe the distribution of lanthanum in the placenta and fetus. Results: Observed by naked eye, the plancenta was dyed into deep blue while there was no sign of dyeing to the fetus in all groups. Under fluorescence microscope, the red fluorescence radiated by EB was observed in placenta but not in fetus. The EB amount in placenta, insonated by microbubbles-enhanced ultrasound of varied MI, was higher significantly than that in MB, US and control group (all P < 0.01) while there was no difference between the latter three groups. And in each group, EB amount was much higher in placenta than that in fetus (P < 0.01). The lanthanum particles deposited in the intercellular space in the syntrophoblast while there was no lanthanum presented in the cytotrophoblast in all groups. Conclusion: Our findings suggest that diagnostic CEU with SonoVue will not increase the permeability of placenta to the macromolecules larger than albumin, although it may affect placenta. © 2009 Elsevier Ltd. All rights reserved.