Coronary neovascularization was studied following grafting of avascular hearts from gestation day-12 (E-12) rat embryos to the anterior eye chambers of adult rats. Volume densities (Vv) of vessels, myocytes, and the extracellular matrix (ECM) after 3-7, 14, 21, and 35 days in oculo were compared to Vv in hearts developing in utero at E-15, E-18, and E-20. The myocardium in both models exhibited similar vessel Vv and capillary developmental stages: (1) clustering of endothelial cells and red blood cells; (2) endothelial cell migration, and (3) tube formation/maturation. The Vv of myocytes increased while that of the ECM remained constant over time. Cross-species grafting utilizing species-specific antibodies determined that the majority, but not all, of the 10-day graft vasculature was of graft origin. Therefore, both de novo growth (vasculogenesis) and sprouting (angiogenesis) were occurring in oculo. Tracer molecules infused into host rats reached the outermost graft vessels only after 10 days in oculo, suggesting a functional link with the host circulation after this time. Thus, we have shown that both models exhibit similar: (1) vascular Vv; (2) shifts in Vv of nonvascular components; (3) stages of neovascularization, and (4) mechanisms of neovascularization. In conclusion, coronary neovascularization occurring in oculo closely mimics normal coronary vessel development. © 1994 S. Karger AG, Basel.