Fujii, Fujita, and Soejima of our laboratory, after a series of experiments with adult rabbits, have already made it clear that retension or mobilization of the marrow blood by the marrow circulation of active substances in the marrow is controlled by contraction or dilatation of the marrow blood vessels, namely, by the vascular actions: and they further have mentioned that no such retension or mobilization is observable in the rabbits blocked of the reticul-endothelial system by indian ink. Therefore, in order to investigate by what mechanism the blocking of the reticulo-endothelial system inhibits vascular actions, the author has performed the following experiment. Namely, by pouring 1% Patent blue, Tripan blue, or Berlin blue dissolved in saline in intact adult rabbits and in the rabbits blocked of the reticulo-endothelial system by indial ink. Splteholz's preparates and tissue specimens of the femur have been made, and the permeability through blood vessels of the dyes mentioned above has been observed: and the following interesting results have been obtained: 1) Any one of these dyes can readily pemeate from venous sinuses of the bone marrow if intact rabbits while none permeates in the case of the rabbits blocked of the reticulo-endothelial system by indian ink. 2) From the above fact, it seems that all the active substances of the bone marrow permeate through venous sinusess into the marrow parenchyma on which they act, at the sametime exerting influences on the marrow nerves; while in the cases whose reticuloendothelial system is blocked by indian ink, that is, when the walls of venous sinus are blocked with indian ink, the permeability of such active substances is inhibited and consequently no action takes place.