With the objective of studying the natural defense mechanisms against traumatic hemorrhages, 20 mice were studies. Some of the large vessels were examined histologically after fixing with formaldehyde, in cases of bleeding, bleeding + hemodilution and controls. In the hemo-diluted group, important alterations in the distribution of the components of the blood stream were confirmed and in one case, there was extravasation of liquid poor in cellular elements. The results show that hemodilution is an unfavorable condition for the natural defense mechanisms against hemorrhage. The repercussion of these experimental results on clinical cases of traumatic hemorrhage is speculated.