Our knowledge of organ ontogeny is largely based on loss-of-function (knockout) or gain-of-function (transgenesis) approaches. However, developmental modulators such as proteins, mRNAs, microRNAs(miRNAs), small interfering RNAs, and other small molecules may complement the above DNA-modifying technologies in a much more direct way. Unfortunately, their use is often limited by the ability of these compounds to cross the placenta and reach physiologically relevant concentrations when administered systemically to the mother. The design of safe and effective techniques to deliver these compounds into the embryo is therefore an area of great scientific potential. In this article we report a new method for introducing developmental modulators into murine embryos by means of direct injection into the heart. Unlike other reported methods that require surgical exposure of the uterus, our percutaneous ultrasound-guided approach allows for the intracardial injection of mouse embryos as early as embryonic day 10.5 (e10.5) and throughout gestation in a minimally invasive manner that largely preserves embryo viability. This system offers a critical advantage over in vitro settings because the effects of any given treatment can be observed without disturbing the native environment of the developing organ. Procedures are described for the delivery and detection of transducible proteins as well as morpholinos designed to block the expression of specific miRNAs within the living embryo.