1. During reaming of the medullary cavity prior to nailing, hydraulic pressure builds up in the cavity which far exceeds that of blood pressure. The peak values for 29 sheep clearly exceeds 2.5 bar. The highest values for individual reamers average 1.3 bar. The differences between the various reamers are only small. Intraoperative measurements on patients have confirmed these results. 2. The reamer acts as a hydraulic piston in a piston sleeve which is filled with a mixture of medullary fat, blood, blood clots and bone debris. The level of pressure depends on the flow rate of the medullary cavity contents between the reamer and the bone according to the gap formula. This is particularly dependent on the gap height which is to the third power in the gap equation. 3. The pressure rise can be most effectively prevented by reducing viscosity of the medullary content using the irrigation-suction technique. The reaming rod is cannulated and is continuously filled with Ringer solution in the distal part of the medullary cavity. This is then drawn off proximally after rinsing the reamer. Experimental measurements on 19 sheep did not produce high pressures, but rather negative pressures. 4. Venting holes in the bone only have little effect because their diameter is too small and local peak values must be assumed during the passage of the reamer. 5. Reaming the medullary cavity less does not prevent pressure increase. Pressure is high even for reamers of small diameter.(ABSTRACT TRUNCATED AT 250 WORDS)