Background/Aims: Since the middle ear cavity plays an important role in middle ear effusion, the goal of this study is to examine the morphological and functional development of the middle ear mucosa in neonatal and adult gerbils. Methods: Gerbils aged between postnatal day 1 (P1) and P26 were used for the experiments. The morphological development of the middle ear cavity was measured by using the celloidin embedding method and scanning electronic microscope. Ciliary motion was determined and calibrated by a photodiode-based measurement system. Results: The area of the middle ear increased significantly from 0.42 ± 0.02 mm² (n = 8) at P1 to 2.6 ± 0.03 mm² (n = 8) at P26. At P1, all regions were covered by densely populated ciliated cells. A significant reduction of the density of ciliated cells was found between P8 and P18 (p < 0.05). The beat frequency at P1 is not statistically different from that of P8 (p > 0.05). However, the beat frequency increased significantly between P8 and P18 (p < 0.05). Conclusion: Middle ear mucosa underwent a significant change between P1 and P18. The cilia morphology and function (ciliary motion) are fully mature at P18. This may explain the high incidence of middle ear effusion in the neonatal period in humans.