The exquisite sensitivity and frequency selectivity of vertebrate hearing are benefits from nonlinear mechanical sound processing. As a result of this nonlinearity, when stimulated simultaneously with two tones, the inner ear produces sounds of still other frequencies, so-called distortion-product otoacoustic emissions (DPOAE). DPOAEs have become a research tool and a clinical tool for evaluating cochlear health status in humans. However, the basic mechanisms of DPOAE generation are not completely understood. My aim is to explore essential characteristics of DPOAE by using the frog inner ear. Two species representative of different stages of evolutionary development of ears have been selected: Lithobates pipiens and Xenopus laevis. I propose to assess the implications of acoustic pathways for DPOAEs by comparing sound transmission through tympanic and extratympanic routes. I plan to explore acoustic interactions between two DPOAE sources, and to evaluate the susceptibility of the basilar papilla to highly noisy environments. Despite morphological and functional differences among hearing organs, understanding frogs´ DPOAE generation mechanisms might yield significant insight into fundamental principles functioning in all vertebrate ears.