1. Intramedullary bone-marrow pressure levels are dependent upon the systemic-blood pressure and the patency of the nutrient artery and possibly other smaller vessels.
2. Intramedullary pressure may be lowered by simultaneously lowering systemic-blood pressure by injection of histamine. The marrow pressure can be increased simultaneously with the systemic pressure by amphetamine.
3. Intramedullary pressure may be lowered while there is a simultaneous rise in the systemic-blood pressure. Adrenalin, nor-epinephrine, and pituitrin produce the response.
4. From the evidence presented, it is believed that the major effect of adrenalin, nor-epinephrine, and pituitrin is produced locally and is independent of systemic-blood pressure. The marked fall in marrow pressure following injection of adrenalin and nor-epinephrine is probably due to vasoconstriction of the nutrient vessels.