Ferric chloride and human thrombin solutions, either individually or mixed, were introduced into the right ventricle or into the auricular veins of 73 rabbits, and the subsequent changes in the pulmonary vessels and auriculer viens were examined by means of light and electron microscopy. Platlet plugs (pseudothrombi) formed immediately after the treatments are aggregation of platlets with apparently intact cell membranes and could be differentiated from the arethentic thrombi in that in the latter there was fibrin invasion of the endothelial cells. Release of the alpha granules from the platlets played a great role in fibrin formation and appeared to occur without disruption of the cell membranes. The endothelial cytoplasms were invaded by fibrin in an early phase of thrombosis and injury to the endothelial cells probably act as a trigger to the further development of the thrombi. So thrombosis pogressed, the platlet cytoplasms were also invaded by the fibrin. As an initiating factor in the development of thrombosis the author emphasizd the role of increased coagulability. ACTA PATH. JAP. 16:1‐36, 19