AbstractThe anatomical distribution of intra‐myocardial collateral arteries that develop from the septal to the other major coronary arteries was studied in dogs following gradual Ameroid occlusion of tftt circumflex artery. The septal artery was cannulated and injected with Batson's plastic compound resulting in a cast of the coronary circulation. Collateral vessels radiated from the septal vascular bed to both the circumflex and anterior descending arteries. The collaterals developed from the entire base‐to‐apex extent of the septal artery and were found on both the right and left sides of the septum. Collateral growth appeared to be more concentrated at the apex of the heart. The anatomical details of septal collateral circulation illustrate the importance of intramyocardial collateralization in the dog, which was thought to exhibit primarily epicar‐dial collaterals.