Immediate reactions to different types of trauma have been the object of several studies recently. It has been shown that pulmonary platelet trapping (PPT) occurs within minutes after both septic shock and soft-tissue trauma. The purpose of this study was to investigate whether hypovolemia induced by hypoperfusion might trigger platelet trapping in the lungs in the same way as soft-tissue trauma. Platelets labelled with indium-oxine were reinfused in anesthetized and mechanically ventilated pigs 4 hours before either induction of standardized hypovolemia caused by bleeding to the amount of 20% of the estimated blood volume (n = 6) or a standardized soft-tissue trauma to the hind limbs (n = 7). Platelet sequestration in the lungs was recorded dynamically by means of scintigraphy for 15 minutes before and 90 min after the start of the trauma and bleeding episodes. Central hemodynamics were recorded using a Swan-Ganz catheter. Soft-tissue trauma induced a marked PPT; in the animals subjected to bleeding alone there was no such effect despite a hemodynamic deterioration of greater magnitude than in the trauma group. The PPT was accompanied by a reduction in the number of platelets and leukocytes in peripheral blood. Our results indicate that immediate trapping of platelets in the lungs after trauma occurs as a response to factors other than those related to simple hypovolemic hypoperfusion.