We have identified a novel hierarchy of human endothelial colony forming cells (ECFC), which are functionally defined by their proliferative and clonogenic potential and in vivo vessel forming ability. Utilizing previously established clonogenic assays for defining different subpopulations of human ECFC, we now show that a hierarchy of ECFC, identical to the human system, can be isolated from the peripheral blood of rhesus monkeys (Macaca mulatta), and that the frequency of the circulating cells varies with age. Endothelial cells derived from rhesus monkey ECFC share a cell surface phenotype similar to human cord blood ECFC, rapidly form capillary like-structures in vitro, and rhesus monkey endothelial lined vessels in vivo upon implantation in immunodeficient mice in an age-dependent manner. Of interest, while ECFC from the oldest monkeys formed capillary like-structures in vitro, the cells failed to form inosculating vessels when implanted in vivo, and displayed a deficiency in cytoplasmic vacuolation in vitro; a critical first step in vaculogenesis. Thus, these studies establish the rhesus monkey as an important preclinical model for evaluating the role and functions of circulating ECFC in models of primate vascular homeostasis and aging.