OBJECTIVE: We characterize rat placenta microstructure in the context of the reduced uterine perfusion pressure (RUPP) model of preeclampsia using the homodyned K-distribution to parameterize envelope-detected signals of ultrasound radiofrequency echo frames obtained in vivo. Preeclampsia is a life-threatening pregnancy syndrome related to abnormal placental tissue microstructure which motivated the quantitative ultrasound-based tissue characterization approach used in this study. METHODS: Ultrasound radiofrequency echo frames against time (or videos) were obtained on 30 and 38 in vivo placentae at gestation day (GD) 14 and 18 respectively, using 9 Sprague-Dawley rats. Preeclampsia-like effects were induced by surgical modification (post GD 14) following the RUPP model, giving a total of 20 RUPP and 18 control placentae at GD 18. The homodyned K-distribution was fit to value distributions of envelope-detected signals of ultrasound radiofrequency echo frames against time, yielding temporal $\alpha$ (scatterer number per resolution cell) and $\kappa$ (ratio of coherent to diffuse signal power) parameters used to characterize the placental tissue microstructure. RESULTS: Visualization of GD 18 $\alpha$ values as a color overlay on B-mode ultrasound video suggested higher values of control compared with RUPP. The mean kurtosis for RUPP was 4.07 ± 0.71 in comparison to 5.08 ± 1.28 for the control using placenta-level kurtosis values (p = 0.0044). There were no significant differences observed in GD 14 placentae, consistent with expectations. Further, we visualized and quantified temporal changes in GD 18 $\alpha$ values with frame-level statistics that support earlier findings. CONCLUSIONS: This study quantitatively characterizes rat placenta microstructure using the homodyned K-distribution and temporal $\alpha$ and $\kappa$ parameters.