Mucus was placed in the middle ears of 11 cats. A needle and stop-cock placed in the bulla communicated with the tympanic cavity. The pressure changes in the needle were transcribed on a flowing graph via a transducer in communication with the stop-cock. Negative pressure developed in each test ear. The average maximum negative pressure was 12 mm Hg with a minimum of 2 mm Hg and maximum of 24 mm Hg. The peak negative pressure, reached in 30 to 60 min. in most ears, occurred in 10 min. in one and in 3 hrs. in another. The qualitative results show all ears developed a negative pressure when mucus was in the tympanic cavity. The quantitative data suggest an average maximum negative pressure of 12 mm Hg reached at about 1 hr.