In this study, intramedullary reaming and nailing were performed following the insertion of pressure transducers in intact tibias and femora. The femur and tibia were instrumented in 12 sheep (group I) and both tibiae in four (group II). The eight procedures of group II were monitored additionally using echocardiography to detect emboli. Intravasation of fat globules was demonstrated in the blood by the Gurd test and correlated with intramedullary pressure and with echocardiographic monitoring in group II. Medullary nailing was found to be always associated with a severe increase in intramedullary pressure, reaching an average of 1126 mm Hg (304 to 1450 mm Hg) in the tibia and of 753 mm Hg (310 to 1126 mm Hg) in the femur during the first reaming procedures. Particle or fat intravasation was greatest during nail insertion. This phenomenon did not depend on the rise in intramedullary pressure. Our findings indicate that fat and bone marrow intravasation occurs during reaming and nailing in long bones. The maximum embolization of marrow contents demonstrated by echocardiography is seen during nail insertion independent of the changes in intramedullary pressure.