Sho-saiko-to, one of the the most frequently prescribed Kampo medicines, is used to treat chronic hepatitis and has shown confirmed clinical efficacy. The present study was performed with respect to heme metabolism to study the preventive effects of Sho-saiko-to against endotoxemia. Endotoxin was injected intraperitoneally at a dose of 6 mg/kg into Sho-saiko-to (500 mg/kg/d, p.o.)-pretreated rats, and its administration clearly prevented the endotoxin-induced hypoferremia. In rats pretreated with Sho-saiko-to, the activity of hepatic δ-aminolevulinate synthetase and cytochrome P-450 level 18 h after endotoxin injection were significantly increased as compared to rats treated with endotoxin alone. Similarly, Sho-saiko-to significantly depressed the endotoxin-induced increase in heme oxygenase activity in liver microsomes. These findings suggested that the extent of shock syndrome caused by endotoxin may be due, at least in part, to changes in heme metabolic disturbance during endotoxemia. Sho-saiko-to may therefore protect rats against lethality caused by endotoxin through its ability to regulate the heme metabolism in septic shock.