By cutting off the posterior roots of the fifth, sixth and seventh lumbar regions of spinal cord as well as of the first sacral segment, on the left side of young dog, the author studied the changes in the bone marrow picture of the lower extremeties comparing that on the right side as the control, and obtained the following results: 1) No marked change can be recognized in macroscopic findings of the bone marrow in the femur and tibia. 2) In the bone marrow of the femur there is a tendency of a slight decrease in the number of nucleated cells as compared with that on the side of the control. 3) In the bone marrow of the tibia, though no difference can be recognized between erythrocytes and neutrophils, in the femur bone marrow erythrocytes have relatively decreased but neutrophils have increased as compared with those of the control. 4) There is a tendency of a decrease in the mitoses of erythrocytes and neutrophils as compared with those on the side of the control. 5) Both erythrocytes and neutrophils as compared with those on the side of the control present a picture of a shift to the left, namely, an inhibition of maturation. 6) Segmented neutrophils have increased in number as compared with the control, indicating an inhibition of the cell outflow from the bone marrow. From these the author arrived at the conclusion that the blocking of parasympathetic nerves primarily inhibits the cell outflow from the bone marrow and accompanying this, cell maturation is also inhibited in the parenchyma of the bone marrow.