INTRODUCTION. The accumulation of atmospheric oxygen starting about 2 billion years ago was permissive for the evolution of complex aerobic metabolic pathways[1,4]. It is thus axiomatic that aerobic capacity defines both a large part of our biology and a divide in the continuum between health and disease[2]. While heritability studies predict a genetic component that accounts for as much as 70 to 90% of the variation in aerobic capacity[3], the genes causative of the difference between low and high capacity have not been defined. In 1996 we started artificial selection for low and high aerobic treadmill-running capacity in rats. The purpose was to create low-capacity runners (LCR) and high-capacity runners (HCR) that could ultimately be developed into contrasting strains for intrinsic (i.e., untrained) aerobic capacity. Here we report the response to selection across nine generations of divergent artificial selection for aerobic treadmill-running capacity.