Protective action of conjugated equine estrogen (CEE) against endotoxin shock was admitted in mice. Male DDN strain mice pretreated intraperitoneally with 2 mg of CEE showed various survival rates according to the interval between administration of CEE and endotoxin injection. Survival rates of the control were 20.4%, but in the group pretreated with CEE they were 0% at 0 hour, 26.3% at 1 hour, 33.3% at 12 hours, 55.7% at 24 hours (p less than 0.001), 10.5% at 36 hours, and 31.2% at 48 hours respectively. Mice pretreated with 2 mg of CEE before 24 hours exhibited excellent resistance to 0.5 mg endotoxin. On the other hand, the hemodynamic effect of CEE was examined in six mongrel dogs, which showed little hemodynamic changes following administration of 10 mg/kg CEE for three hours observation. We speculated that the anti-shock action of CEE was due to the modification of vascular response to vasoactive substances, on the one hand it augmented the vascular contractile response to vasopressors and on the other hand it lessened the hypersensitive vasoconstriction under shock state.