AbstractMicrocurie amounts of p32, Na22, Fe59, and Cl36 in normally occurring compounds were injected directly into the vitelline vessels of developing chick embryos at stage 24 (Hamburger and Hamilton, '51). The surviving embryos were sacrificed at stage 41. Hearts were removed and examined intact or sectioned for later examination. Coronary vessels were identified, classified according to size (large, diameter = > 80 μ; medium, diameter = 40–80 μ; small, diameter < 40 μ) and counted. There was a significant increase in the numbers of small and medium vessels which had developed in the presence of gamma radiation emitted by Na22‐ and Fe59‐compounds. The p32‐ and Cl36‐containing compounds did not affect the numbers of vessels of any size. The resulting increased vascularity is shown to be a matter of a distinct increase in numbers of blood vessels as opposed to a simple alteration of the normally occurring vessels.