Large doses of estrogens have a profound effect on the bone marrow of adult dogs. The initial reaction is a great increase in the number of neutrophilic elements in the bone marrow. These neutrophils are released into the blood stream causing a marked rise in the total white cell count. This is followed by congestion of the bone marrow and a destruction of the white cell elements in these marrows. Congested areas and locations formerly occupied by white cell elements are replaced by edema, leaving erythroid elements intact. This accounts for the marked drop in the total white cell count in the blood stream. This is followed by destruction of remaining erythroid elements in the bone marrow and replacement by "edema" until a stage is reached where practically no cells can be found in the marrow.