||Earlier findings have demonstrated that pretreatment of Herba Cistanches, a Yang-invigorating Chinese tonic herb, stimulated the ATP-generation capacity (ATP-GC) in mitochondria isolated from rat hearts ex vivo. The enhancement of mitochondrial ATP-GC by Herba Cistanches was associated with the induction of mitochondrial uncoupling and the protection against ischemic/reperfusion (I/R) injury in rat hearts. This thesis study aimed at (1) identifying the active fraction(s) in Herba Cistanches and (2) further investigating its biological activities and their underlying mechanism. Using ATP-GC stimulation as an activity monitor, a semi-purified fraction of Herba Cistanches ethanol extract (HCF1), which contained relatively non-polar ingredients, was isolated and subjected to further investigations. Pretreatment of HCF1 was found to increase ATP-GC and induce mitochondrial uncoupling as well as cellular antioxidant response in H9c2 cardiomyocytes, with the latter being assessed by the enhancement of glutathione reductase (GR)-mediated glutathione redox cycling. It was hypothesized that the increase in mitochondrial reactive oxygen species (ROS) production associated with the stimulation of ATP-GC can trigger cellular responses including mitochondrial uncoupling and GR-mediated glutathione redox cycling, with resultant protection against oxidant injury. To test this hypothesis, the effect of HCF1 on mitochondrial ROS production was examined. HCF1 pretreatment increased mitochondrial ROS generation in H9c2 cardiomyocytes. The removal of mitochondrial ROS by antioxidants (dimethylthiourea and Trolox) abrogated the HCF1-induced cellular responses and the associated cytoprotective effect. Studies using specific inhibitors of reduced glutathione synthesis and regeneration suggested the important role of GR-mediated glutathione redox cycling in the cytoprotection afforded by HCF1. Results obtained from animal studies showed that HCF1 pretreatment protected against myocardial I/R injury in rats. In conclusion, HCF1, an active fraction of Herba Cistanches that contained relatively non-polar ingredients, could enhance mitochondrial ATP-GC in H9c2 cardiomyocytes and mitochondria isolated from rat heart. The associated increase in mitochondrial ROS production induced mitochondrial uncoupling and GR-mediated glutathione redox cycling in H9c2 cardiomyocytes. The enhancement of glutathione redox cycling by HCF1 protected against oxidant injury in cardiomyocytes. The long-term intake of HCF1 can offer a prospect for the prevention of cardiovascular diseases.