Auditory nerve responses to condensation and rarefaction clicks were recorded from the external ear canal of cats using a closed acoustic system. Repeated control recordings from both ears formed a baseline for each of four animals used in this study. After a baseline had been established, the Eustachian tube on one side was ligated and serial recordings of N1 responses were performed for up to 140 days postligation. By comparing the shift which occurred in the N1 latency-vs-click level plots after ligation, the equivalent hearing loss was determined. In all cases where the Eustachian tube was successfully ligated, the loss was progressive for the first 20 days, then usually showed some transitory improvement. The loss stabilized after 60 days, varying from 15 to 40 dB in different animals. In addition to N1 recordings, serial tympanograms were also measured. These indicated negative middle ear pressure in the first two days postligation and the presence of middle ear fluid by one week postligation.