Endoneurial fluid pressure (EFP) was measured in the facial nerve in rabbits and guinea pigs using a servo-controlled micropipette system. EFP in the tympanic segment was 5.3 +/- 1.2 cm H2O in rabbits and 4.1 +/- 0.9 and 4.2 +/- 0.9 cm H2O in the tympanic and mastoid segment respectively in guinea pigs, while EFP in the sciatic nerve was around 2 cm H2O. The higher pressure in the facial nerve may be related to the proximo-distal flow of the endoneurial fluid in peripheral nerves.