AbstractObjectives/Hypothesis:A third window in the vestibular apparatus has been investigated in both animals and humans, specifically in superior semicircular canal dehiscence. There are as yet no animal model studies of the effect of a third vestibular window of the posterior semicircular canal.Study Design:Original basic research study.Methods:A fenestration was drilled in the bony labyrinth over the posterior semicircular canal, preserving the membranous labyrinth, in seven healthy, 6‐month‐old, fat sand rats (total 10 ears). Auditory brain stem responses to low‐ and high‐frequency acoustic stimuli delivered by air conduction and bone conduction were recorded before and after fenestration.Results:On the preoperative auditory brainstem recordings, air‐conduction thresholds to clicks and tone bursts averaged, respectively, 6.5 dB and 7.5 dB, and bone‐conduction thresholds, 8 dB and 4.5 dB. Postoperatively, air‐conduction thresholds averaged 14.5 dB, and bone‐conduction thresholds 10.5 dB and 5 dB. The change in air‐conduction thresholds was statistically significant (P < .01), whereas the bone conduction thresholds remained unchanged.Conclusions:A vestibular third window in the posterior semicircular canal decreases the sensitivity to air‐conducted sound stimuli, raising the air‐conduction threshold. There is no change in the bone‐conduction threshold. These findings agree with the theoretical model and clinical findings. Laryngoscope, 2010