SummaryMale Wistar rats received intraperitoneal injections of adriamycin (4mg/kg body weight/day) and/or riboflavin-butyrate (20mg/kg body weight/day) for 6 consecutive days. Cardiac mitochondria were then prepared for our present experiment. The combined use of riboflavin-butyrate with adriamycin was evaluated for reduction of lipid peroxide formation in rat cardiac mitochondria. In order to find the mechanism of the effect of riboflavin-butyrate, the glutathione peroxidase glutathione reductase system was examined. Adriamycin reduced the glutathione reductase activity in rat cardiac mitochondria, but did not affect the glutathione peroxidase activity. The mitochondrial content of flavin adenine dinucleotide, a prosthetic group of glutathione reductase, was greatly reduced and apoprotein of glutathione reductase also de creased. The administration of riboflavin-butyrate with adriamycin in creased flavin adenine dinucleotide and glutathione reductase activity. These results suggest that exogenous administration of riboflavin-butyrate is capable of reducing lipid peroxide by both enzymatic detoxification through glutathione reductase and non-enzymatic detoxification due to direct reaction with lipid peroxide.