Seventeen mongrel dogs were subjected to ligation of the circumflex branch of the left coronary artery. At intervals from 10 to 300 minutes after ligation, 10 ml of a sodium fluorescein and colloidal thorium hydroxide solution were injected via an intra-arterial catheter placed distal to the ligature. Up to 20 minutes after ligation this tracer solution evenly perfused the left ventricular wall including the posterior papillary muscle (PPM). During 25 to 50 minutes there was a progressive patchy loss of perfusion into the PPM although the sub-epicardial region was evenly perfused. After 60 or more minutes of ischaemia it was not possible to inject this tracer solution into the subendocardial region. Electron microscopic examination indicated that this loss of vascular competence was possibly due to vasospasm of some vessels in the marginal zone between the perfused and non-perfused areas and was unlikely to be due to intravascular thrombosis, endothelial cell swelling, or to compression of vessels due to cell or tissue oedema.