The development of the middle ear cavity has posed many problems in research on embryology. Particularly, the development and resoption of the mesenchyme from the middle ear does not seem to be fully understood. The role of the middle ear mesenchyme not only seems to be related to the epithelial migration and differentiation of the middle ear space, but it is possible that the excess of the mesenchyme may predispose to the development of otitis media or may inerease its severity. The purpose of this study was to describe the development and resorption of the murine middle ear mesenchyme and to provide anatomical basis for the future animal studies concerning the pathogenesis of otitis media. Sixty three specimens, aging from gestational day 16 to postnatal day 21, were investigated through the light microscopic observations. The present study also included a quantitative investigation for population density at different gestational and postnatal ages. At gestational day 17, the mesenchymal tissue in the middle ear cavity began to increase and at birth most of the middle ear cavity was filled with the mesenchyme. The middle ear mesenchyme disappeared at postnatal day 11 through 13, but it still remained in the attic area. The mesenchyme showing high cellularity at around birth became looser and less cellular with increasing age.