Spinal nerve roots including the dorsal root ganglion (DRG) often are mechanically deformed in connection with degenerative and traumatic conditions of the spine. However, the pathophysiology underlying various functional changes, including pain production, in such conditions is incompletely known. In this study, the tissue fluid pressure in the DRG of L5 nerve roots of rats was measured before and after compression. Normal values were found to be 3.7 +/- 0.3 cm H2O (2.7 +/- 0.2 mm Hg). After mechanical compression, the endoneurial fluid pressure in the ganglia rose to 9.6 +/- 1.7 cmk H2O (7.1 +/- 1.2 mm Hg) (P less than 0.001). Histologic examination revealed edema and haemorrhage in the endoneurial space of the DRG. Pressure increase in the DRG as a result of mechanical deformation by, for example, a herniated disc might be expected to reduce blood flow to the sensory nerve cell bodies in the DRG. This may be a mechanism underlying the production of nerve root pain, which previously has not been described.