Eustachian tube function (ETF) was evaluated in 22 ears of 18 chinchillas using the inflation-deflation and forced-response protocols. The results of the ETF tests showed an unusual function in the chinchilla. Of the 22 tests, one indicated a patulous eustachian tube (ET) and 20 suggested a semipatulous ET. This was evidenced by a passive, pressure-dependent, complete reduction of applied positive or negative ME pressures in these ears. This phenomenon has no counterpart in the ETF of rhesus monkeys or humans. Discrete changes in the continual leakage of applied pressures, indicative of active tubal dilation, were only observed during tests in four ears. Passive tubal resistance was similar to that of humans, though all values were characterized by large variances. Active tubal resistance could only be defined for four of the test ears. The ET of eight ears constricted with swallowing. In nine test ears, neither constriction or dilation was observed during the forced-response test, and four tests showed dilation of the ET with swallowing. These results show that the ETF of the chinchilla is radically different from that of humans or monkeys.