Hydrogen peroxide (H2O2) kills the rat in 2 or 3 mn by gaseous emboli due to oxygen formation by blood catalyse activity. Venous return and pulmonary circulation are blocked by numerous bubbles. 2) Letal dose (DL100) H2O2 averages 5 +/- 2 mg 100 g-1 BW by subcutaneous route. Intraperitoneally, 60 mg 100 g-1 BW H2O2 are well tolerated, only 10% rats dying by emboli; 40 mg H2O2 injected in a gas pocket are also well tolerated. 3) Access of the blood stream of oxygen bubbles is limited by some anatomical peculiarities of the venous capillaries, notably hepatic sinusoids.