The effects of standardized blood loss on the platelets, as well as on coagulation, fibrinolysis and complement parameters were studied in the pig. A simultaneous fall in arterial and venous platelet counts was demonstrated during the initial phase of blood loss, with a concomitat rise in pulmonary vascular resistance. After exsanguination to the extent of about 40% of the blood volume, the arterial platelet counts increased significantly, whilst the pulmonary vascular resistance, which was negatively correlated with the arterial platelet counts throughout the experiment, fell. Venous platelet counts, on the other hand, remained low during this experimental period and were correlated with cardiac output, but not with pulmonary vascular resistance.