DNA synthesis and mitotic activity in coronary collateral arterioles was assessed in dogs at different time intervals after gradual ameroid constriction of the left circumflex coronary artery. Labeling of nuclei and the mitotic index were highest at 3 weeks after implantation of the constrictor and gradually declined thereafter. Labeling persisted for at least 8 weeks but radioactive DNA was not found 12 months after coronary artery constriction nor in controls or animals with sham operations. Proliferative activity was, at any time interval, highest at the level of the smallest diameters of the collateral vessels. Labeled nuclei and mitoses were found in endothelial, medial, and adventitial cells. Myocardial mesenchymal cells also incorporated tritiated thymidine. The data provide evidence that after constriction of a major coronary artery, the coronary collateral vessels enlarge by an active growth process that follows the basic laws of cell kinetics.