The history of developmental and genetic analysis in the mouse has made it the model of choice for studying mammalian embryogenesis. Presently lacking is a simple technique for efficiently analyzing early mouse mutant phenotypes in utero. We demonstrate application of a real-time imaging method called ultrasound backscatter microscopy for visualizing mouse early embryonic neural tubes and hearts. This method was used to study live embryos in utero between 9.5 and 11.5 days of embryogenesis, with a spatial resolution close to 50 microns. Ultrasound backscatter microscope images of cultured embryos made it possible to visualize the heart chambers. This noninvasive imaging method was also used for analyzing a neural tube defect. The midhindbrain deletion associated with a null mutation of the Wnt-1 protooncogene was easily recognizable on ultrasound backscatter microscope images of 10.5- and 11.5-day embryos. Computer-generated volumetric renderings of the neural tube cavities were made from three-dimensional image data. This allowed a much clearer definition of the Wnt-1 mutant phenotype. These imaging techniques should be of considerable use in studying mouse development in utero.