Progressive narrowing of the left circumflex coronary artery, leading to complete occlusion within about 3 weeks, was produced in dogs by the ameroid technique. The development of intercoronary anastomoses by growth of pre-existing channels of arteriolar size was studied on the cytological, histological, histochemical, and arteriographic level. Several characteristic stages in the transformation of a small arteriole into a small artery are described.