Mechanisms leading to the obstruction of the microcirculation in sinusoidal obstruction syndrome (SOS) have been unclear. Because this occurs at the onset of disease, this is a potential key target for therapeutic intervention. Rats were treated with monocrotaline with or without continuous intraportal infusion of glutathione and were studied at 0.5, 1, 2, 4, 6, and 10 days after monocrotaline treatment with the use of in vivo microscopy and transmission electron microscopy. Sinusoidal perfusion decreased from days 1 through 10 with a nadir on day 4. At 12 h, numerous swollen sinusoidal endothelial cells (SECs) were observed. Subsequently, red blood cells penetrated into the space of Disse through gaps between and through swollen SEC and dissected the sinusoidal lining away from the parenchymal cells. Sinusoidal blood flow was obstructed by an embolism of aggregates of sinusoidal lining cells, red blood cells, and adherent monocytes. All changes were prevented by glutathione infusion, notably the initial swelling of SEC. SOS is initiated by changes in SEC. Microcirculatory obstruction is due to dissection of the sinusoidal lining, followed by embolization of the sinusoid by sinusoidal lining cells, compounded by aggregates of monocytes adherent in the sinusoids. Glutathione prevents SOS by preserving an intact sinusoidal barrier.