In vivo behaviour of Indium-111-labelled platelets were followed simultaneously in the lungs, liver, spleen and kidneys in 16 adult sheep which were exposed to intravenous injection of endotoxin (10 micrograms/kg body wt.). The effects on the respiratory function and the central hemodynamics were also followed. Eight sheep received, 30 min after the endotoxin challenge, continuous intravenous infusion of low molecular weight dextran during 4 h while the other eight animals served as controls. A marked pulmonary and hepatic platelet sequestration was observed during and just after the endotoxin infusion and was followed by a marked platelet disaggregation within 30 min in both the groups. Three hours after the endotoxin a second wave of platelet sequestration occurred in the control animals, both in the lungs and in the liver, but no such increase was seen in the low molecular weight dextran-treated group. Within the kidneys only minor changes occurred during the experiment period. In the spleen, however, there was a decrease in platelet sequestration after endotoxin in both groups. Less marked hemodynamic and respiratory alterations occurred in the dextran-treated group compared to the controls. It was concluded that low molecular weight dextran decreases sequestration of platelets in the lungs and in the liver of sheep during endotoxic shock.