An inverse relationship between initial level of physical capacity and the magnitude of response to training is termed the principle of initial value. We tested the operation of this principle under experimental conditions of minimal genetic and environmental variation. Inbred rat strains previously identified as genetic models of low (COP) and high (DA) intrinsic (untrained) exercise capacity were trained for 8 weeks on a treadmill using two disparate protocols: 1) a relative mode where each rat exercised daily according to its initial capacity, and 2) an absolute mode where both strains received the same amount of training independent of initial capacity. Response to exercise was the change in running capacity as estimated by meters (m) run to exhaustion before and after training. When trained with the relative mode COP rats gained 88 m (+21%; NS) whereas DA rats increased distance run by 228 m (+36%; p <0.001). When each strain trained with the same absolute amount of training, the COP strain showed essentially no change (-6 m, -2%) and the DA strain gained 325 m (+49%; p <0.009). Differences in response to exercise between the COP and DA could not be explained by body mass differences, oxidative enzyme activity (citrate synthase or ATP) or spontaneous behavioral activity. Our data demonstrate that genetic factors causative of high response to exercise are not uniquely associated with genetic factors for low intrinsic capacity and thus are not in accord with the principle of initial value.