A brief résumé will be given of our method of direct measurement of acoustic impedance of the ear in experimental animals with graphs of average impedance curves for guinea pigs. The acoustical impedance of the ear of a single animal is not fixed, but quite variable, particularly in the range of best auditory acuity between 2000 cps and 7000 cps. It appears possible that these impedance variations are controlled by the animal. Experiments will be cited showing the role of the double‐chamber configuration of the middle‐ear cavity in producing impedance changes. Results of some experiments demonstrating the possible effects of tensor tympani and stapedius muscles upon the acoustical impedance of the ear will be given.