We evaluated cardiac muscle development in the absence of hemodynamic work load but in the presence of host factors including blood vessels, nerves, and circulating neurohumoral agents by transplanting 12-day fetal rat ventricle into the anterior eye chamber of adult host rats. Implants were studied by electron microscopy at intervals from 1 to 14 weeks in oculo. For comparison with myocardium developing in oculo, 12-day fetal tissue and 3-, 8-, and 28-day-old normally growing rats were also studied. At 1 week in oculo, myofibrils were laterally located and more frequent than in the 12-day fetus. Fibrils had clear Z bands and H bands, but no M bands. At 10 days in oculo (comparable to birth in normally growing animals), myocyte mitoses were present and tritiated thymidine autoradiography revealed many labeled myocyte nuclei. By 5 weeks in oculo, cells were filled with mature myofibrils with clear M bands and lateral connections between adjacent Z bands. However, myofibril bundles sometimes coursed at sharp angles to each other within single cells. Except for the relative lack of fibrillar polarization and small cell size, ultrastructure of myocytes developing in oculo for 5 or more weeks appeared very similar to myocytes developing in normally growing rats. By 10 weeks in oculo, when in situ growing hearts are clearly in a hypertrophic phase of growth, no mitoses or tritiated thymidine-labeled nuclei were present in myocytes, although labeled nonmyocyte nuclei were present. Morphometric evaluation revealed no change in myocyte diameter or nuclear-to-cytoplasmic ratio from 1 to 3 weeks in oculo, consistent with continued hyperplastic growth. Binucleated cells were present by 3 weeks in oculo and later, and the cytoplasm per nucleus increased fourfold between 3 and 5 weeks in oculo, suggesting conversion to hypertrophic cell growth. We concluded that cells proliferated and differentiated in the absence of a hemodynamic load, but that polarized alignment of myocytes and myofibrils was incomplete.