The results of clinical and animal studies suggest that a short term period of moderate secondary carnitine deficiency, in and of itself, does not have a major effect on the cardiac contractile function, although substrate oxidation may be altered. However, with longer durations of carnitine deficiency, alterations occur within the heart that may result in impaired contractile performance, particularly at high workloads. At this point, the mechanisms responsible for the cardiac depression are uncertain. We hypothesize that the alterations in substrate metabolism produced by the carnitine deficient state results in inadequate ATP production under high workload conditions which result in impaired cardiac contractile performance. Carnitine deficiency may also induce a number of changes in gene expression of key enzymes required for normal cardiac contractile function and metabolism.