The gerbil ear is not mature at birth. The sound‐conducting apparatus as well as inner‐ear structures continue to develop over a period of at least 16 to 20 days postparturition. This circumstance allows a unique opportunity to examine relationships between morphology and function in the auditory apparatus. Mongolian gerbils were vitally perfused at various ages ranging from 20 min to more than 23 days. Serial sections were done and observations made on the developmental status of the external, middle, and inner parts of the ear. Cochlear potentials were recorded from some animals. The presence of reflex activity to sound was noted. Our observations indicate that the earliest reflex response to sound (whole body) occurred at 16 days. Electrical activity could be recorded from the round window niche by day 14. Maturation of the organ of Corti was questionable by day 10, and thelimbusandspiral sulciwere not well differentiated. By day 16, all parts of the organ of Corti appeared well developed. Ossicular ossification was almost complete by day 16, but mesenchyme was still present in the region of theincudomalleolarjoint. [Work supported by NIH.]