An investigation was made into some of the hemodynamic properties of bone. The parameters studied were bone marrow and medullary venous pressures. The resting level and changes in the two pressures were measured under autoperfusion and constant-flow perfusion conditions. Changes were induced either by drugs or by alteration in blood flow. Marrow pressure was measured with a new tonometric technique. A specially constructed miniature pressure transducer was utilized as the tonometer. Trauma-induced hemorrhage of the marrow was avoided. The marrow pressure was measured across an intact endosteal membrane. The pressure of the normal bone marrow and the medullary venous pressure in the anesthetized dog were nearly equal, approximately twenty millimeters of mercury and about 17 per cent of systemic blood pressure. These values were found whether measured simultaneously or separately in comparable groups of dogs under a variety of conditions. Induced changes in the two pressures produced either by changes in perfusion rate or by drugs were also nearly equal and in the same direction.