The characteristic findings in the pathological picture presented by 60 dogs when administered large doses of estradiol benzoate subcutaneously in an oil vehicle are: thrombocytopenia, aplastic anemia, which follows a brief latent period, intestinal tract haemorrhages, haemorrhagic lesions occurring principally in the mucosa of the jejunum, followed by prostration and death generally within 1 to 3 weeks. Irregular findings are leucocytosis, purpuric skin lesions, visceral haemorrhages and pyoid bone marrow. The total circulating hemoglobin was determined in 9 dogs before and after the anemia was established, thus verifying the total hemoglobin loss. The blood loss through intestinal haemorrhages during the observation period was determined by benzidine test. The quantity of hemoglobin recovered from the feces in the form of hematin varied between 30 and 70% of the total hemoglobin which disappeared from the circulation. In order to ascertain the degree of intra-organic blood destruction the bilirubin eliminated in the urine of dogs with gall-bladder renal type of fis¬tula was determined daily and these values compared with those obtained for normal animals. The blood destruction as measured by this method was practically nil. An attempt was made to determine whether the presence of large quan¬tities of blood in the intestine might exert a paralyzing effect on the regene¬ration of hemoglobin in dogs rendered anemic through haemorrhage. Ne¬gative results were obtained. Estradiol benzoate administered to dogs during the period of hematic regeneration which occurs in the recovery stage of ane-mia produced by acetyl-phenyl hydrazine demonstrated that the estradiol in¬terferes but little when the regenerative processes have already begun. Repeated transfusions given during the stage of acute anemia practi¬cally do not affect the period of survival. The authors conclude that the main action of estradiol benzoate on the blood physiology of dogs is still obscure. The anemia is not produced by the intra-organic destruction of hemoglobin by known processes and doubtless the blood loss through the intestinal haemarrhages constitutes an important ac¬cessory factor in the mechanism of bringing out this anemia.