AbstractThe structure and function of the middle ear is considered inGekko gecko(Gekkonidae) andEublepharis macularius(Eublepharidae). Middle ear anatomy is described in detail forGekko, and minor points of difference inEublepharisare indicated. By use of the alternating potentials of the cochlea the peripheral sensitivity function was determined, and then the contribution of the middle ear to this function was assessed by cutting the columellar shaft and removing the more peripheral structures. The sensitivity was then determined by presenting the sounds through a tube sealed over the oval window. This method provides known sound pressures at the columellar footplate and avoids simultaneous stimulation of the cochlea by way of the round window, which other forms of aerial stimulation would produce. This procedure reveals the performance of the middle ear as a mechanical transformer without complicating side effects. The transformer action was indicated as improving peak sensitivity by about 35 db inGekko geckoand by about 53 db inEublepharis macularius.The operation of the middle ear was further analyzed by the use of mechanical vibrations first applied to the surface of the tympanic membrane just over the pars inferior with the ear intact, and then to the stump of the columella after this ossicle was separated. The basic form of the sensitivity function persists in these vibratory types of stimulation, which is taken to mean that this form depends in large measure on the cochlea itself.Two processes are considered as achieving the force amplification required for impedance transformation, which is the principal function of a middle ear. One is a lever action of the columellar system, and the other is the hydraulic action arising from the difference in areas between the surfaces of the tympanic membrane and the columellar footplate. No specific evidence for lever action was obtained in this study. Measurements of areas of the tympanic membrane and columellar footplate indicated fairly large hydraulic ratios in these gekkonoid species. The evidence shows that the efficient transmission of sounds by the middle ear contributes in a significant way to the high degree of auditory sensitivity of these lizards.