Small fragments of embryonic hearts were transplanted either alongside or into a skeletal muscle (soleus) of an adult host. The implanted tissue grew, and survived for at least 6 months after transplantation. The graft was well vascularized and established a network of blood vessels that communicated with the host's circulation. This communication appeared to be established by the proliferation of blood vessels from the graft into the host tissues. The grafted tissue was rhythmically active and the rate of these contractions was similar to that of adult rat hearts. The frequency of the spontaneous contractions could be modified by acetylcholine. Exposure to acetylcholine lead to a reversible slowing of the rate of beating. The presence of gap junctions in the transplanted tissue was revealed by visualizing connexin 43 with a specific antibody. During early periods after grafting the gap junctions were scattered within the graft but over time they became aligned into rows, to prepare for the formation of intercalated discs. Thus embryonic heart grafted into, or alongside skeletal muscle is able to acquire a considerable degree of differentiation.